Sorry non-fans, if you aren't familiar with both you'll be lost because this story takes the premise that BotP takes place decades after Gatchaman in the same universe and doesn't spend a lot of time on the back stories of either except for the specific character info needed for the stories.
If you are a non-fan and are wondering why putting these shows into the same universe is so cool a bit of history so you won't feel left out. Way back in the early '70s Gatchaman was an anime about G-Force, a team of young people who each wore a costume with a different bird theme and traveled in a ship called the Phoenix that could transform into fire. Ken, Joe, Jun, Jenpei, and Ryu defended the Earth from invasion by giant animal themed mecha. It was violent, bloody, tragic, and way ahead of anything American TV would produce for a very long time to come.
Sandy Frank bought the rights to it for American distribution but knew going in that there was no way he could market this to kids (since of course being animated it HAD to be for kids, right?) and set about cutting out anything visual that would offend the censors. This left him with maybe 15 mins of footage max for some episodes even counting the credits. He then gave the untranslated footage to scriptwriters and had them write original stories based on it making use of reusing footage and new animation to fill the holes. The new animation consisted of a robot named 7-Zark-7 who narrated and commented on the show. The difference in quality between the original footage and the new stuff was pretty noticeable and Zark could be really annoying but on the whole it worked. What was produced was Battle of the Planets in which Mark, Jason, Princess, Keyop, and Tiny defend the Earth and its colony worlds from invasion from giant animal themed mecha. It has its problems and I'm really not sure if it's possible to fall in love with it as an adult but it was the show that got me hooked on anime and I love it still.
The characters had obvious similarities, after all the visuals were mostly the same, but there are key differences in their histories and personalities. For instance Jenpei was an orphaned human kid while his counterpart Keyop was the result of a not entirely successful experiment to grow genetically enhanced people in a lab. Joe's dad was a mobster killed in a hit and he wanted revenge while Jason's was an officer with the Earth defensive forces who had died in the line of duty.
To make things more confusing Ted Turner took the same Gatchaman footage and produced a show called G-Force where the names were mostly changed AGAIN. I've only seen one episode of G-Force so I can't comment on it. You can find extensive comparisons between the shows on Wiki.
So, on to the recs if you are still with me.
Title: On My Watch
Summary: The newly appointed Commander of GForce reads some old reports, and makes a vow.
First up is Mark, the Eagle, leader of the team and his thoughts on Ken.
Title: On My Watch II: Condor
Summary: In this one, Jason contemplates another Condor.
Second in command Jason's turn and this is the one that caused me to decide to recommend the series. The first story is decent but unremarkable except for the concept but in this story the characterization really shines and it's one of the better Jason character study fics I've seen.
Title: On My Watch III: Swan
Summary: The third in this series. The Swan reflects.
Princess was my favorite when I was a teen watching this for the first time. This one takes place several years after the first two and is a continuation fic for the original series as well as a great character study.
Title: On My Watch IV: Swallow
Summary: The fourth in the “My Watch” series. The Swallow reflects.
Keyop, the youngest of the team has his turn. I love anything well written that doesn't relegate him to comic relief side-kick status. It looks to be the last of the series as this was written in November 1996 and so far there isn't one for Tiny.