Why was Gaeta after almost 8 years on Galactica only a Junior Lieutenant?

Why was Gaeta after almost 8 years on Galactica only a Junior Lieutenant while others (for example Kat) got promoted quite rapidly all the way up to Captain?

Same question could be asked about Hot Dog who started pilot training together with Kat and who was an equally good pilot.

Maybe it just me but I always thought on Mr Gaeta as a square peg in a round hole. I just got the impression that while he got on with his colleagues, he did quite fit in. Due to his intelligence, he would have been invaluable in his role in CIC and therefore wouldn't have been moved about if he had voiced any frustrations about his lack of advancement or utilisation of his skills he perceived that was being under-used. Of course this is speculation on my part as his character wasn't developed all that fully.

In contrast to Kat, I got the impression she was a people person that was respected but also a leadership type as seen with her competition with Starbuck. I could imagine that one of the reasons for her rapid promotion to give healthy competition in the officer ranks to keep them on their toes. Again, that is speculation on my part.

When applying the US Navy rules, promotions from Ensign (O-1) to Junior Lieutenant (O-2) and from Junior Lieutenant to Lieutenant (O-3) are time-in-grade and time-in-service based.
Basically, after eighteen months as Ensign you will be promoted to Junior Lieutenant and after two more years you will very likely be promoted to Lieutenant (assuming you don't do anything stupid and meet all the requirements.)

"Promotions among the officer corps in the military start off typically as automatic by simply putting in time in rate and meeting the standard.  You have to really do something horrible to not meet the typical "heartbeat" promotions from O-1 to O-3.  Driving under the influence, committing a felony, failing training programs, or not meeting the minimum standards of the military can be one of a few things can prevent you from being promoted." (https://www.thebalance.com/military-commissioned-officer-promotions-4055887)

Things get more demanding to make the promotion from Lieutenant to Lieutenant Commander (O-4, BSG rank: Captain)

It seems in the Colonial system is quite similar to that:
Lee Adama was Captain after roughly 5-6 years in service.
Sharon "Boomer" Valerii was Junior Lieutenant after less than 2 years and she was clearly a "screw-up": bad landings, sexual relationship with enlisted personal,... she had enough deficiencies and did enough stupid things to actually not being promoted at all.

Gaeta not being promoted to full Lieutenant must have been an oversight by the production staff and writers, otherwise it makes no sense.
Kat and Starbuck were both somewhat trouble makers and loud-mouths. Being a natural pilot or competitive is not a qualification for a higher rank. Being responsible, reliable and having leadership qualities are the requirements.
Gaeta did his job well enough (until he started the mutiny) and after 5 years on Galactica (before the Cylon attack) should at least have been promoted to full Lieutenant - and that long before the war had started.
Also, CIC is where the command staff is, not out in a Viper.

Given their service time both, Kat and Hot-Dog, should have been no more than Lieutenants.
And that Kat (CAG by the time of "Occupation") got promoted to Captain before Helo (XO by the time of "Occupation" - and with Tigh "retired" Helo was not just "acting XO") simply can't be right...

And then there were all those Captains with seniority over all of the above mentioned that were just sidelined (Capt. Kelly, Capt. Birch,...)

When it came to ranks they screwed things up quite a bit. That's what happens when you "wing" it...

That's what happens when you "wing" it...

...

Yeah, I'm sure it ruined the show for millions.

It's one of those things that are so low on my priority list that it doesn't even rate an 'I don't have a frak to give'.

All stories have discrepencies where they do not match reality. What I love about stories is not the idea that the creator is supposed to build this unassailable world that completely convinces me that it is real- but rather the fact that the story-teller is really only offering me a framework which I have to fill in with my own imagination.

All 'flaws' in a story are really a measure of my own imagination's limitations IMHO.

As Lee had said, they had turned from a civilisation into a gang on the runs, making and breaking the rules as they went along.

Of course, they had to keep up morale and discipline which would result, amongst other things, rewarding service time, but given their situation, it is possible for them not to be so stringent about enforcing it, considering that Mr Gaeta had left the military for a time on New Caprica as well as Hotdog just joining after the Fall and being the Can't complain type, he may also have been a bit naive about that part of the service and wouldn't be aware of it. There may be also the fact that given the lack of opportunities and advancement, apart from filling posts due to deaths, that it would be beneficial to keep a line between the upper and lower ranks, particularly as a way to reward the 'favourites'.

Of course, you could be correct that it is an obvious flaw, but the beauty of the series is it's ambiguity, where within reason, you can have an explanation for parts of the series that falls under plot-holes.

It may not be important to telling a story, but if you go out to create a hardcore, in detail military SF show (or novel) it is advisable to do a bit of research and work out a framework of rules and follow them. It is also advisable to work with timeline tables and make the writers stick to them. They actually did a fairly good job during season 1 and the first half of season 2, but by season 3 things started to fall apart

All the regulation violations that happened after the Cylon attack could be explained by the need to be flexible (like seen on Galactica) or the commanding officer having gone totally rouge (like Cain, who elevated herself to dictator and executioner in chief).
Though at least until the election of Baltar for president Adama and Roslin tried to uphold the old system and do things as much by the book as possible. But after president Baltar it seems the book got tossed out of the airlock.
For example, there were plenty of more senior and higher ranking officers than Helo to take over as XO, but it seems Adama liked and trusted him so he put him in charge instead of Capt. Kelly, who was 3. in line for command on Galactica at that time. Or Kelly just didn't want that position... but in that case there would still have been plenty of other officers with higher ranks and more seniority than Helo, so it's back to argument no. 1: Adama liked him best for the job... or for who Helo was married to: Adama had developed a father-daughter relationship with Sharon that was even closer than the one he had with Kara (I seriously doubt that he hat teatimes and philosophical discussions with Kara).
An even better example of this cronyism is Dee, who goes from Petty Officer to Junior Lieutenant and XO of Pegasus because she is engaged/married to Adama's son.

But for the time before the Cylon attack things should have been more consistent. And Gaeta not being full Lieutenant by the time of the attack falls into this category.