Thanks to Fantasy Leader, somehow I'm inspired to write this. I kind of think that both series are great in every respect and not exactly much of a Gingaman clone (but the falling older brother was carried over for some reason, Lost Galaxy could have done without it and just made Leo with another story), I think we can point out some differences:
Rangers: The Gingamen were descendants of the original Gingamen who fought space pirates who used mystical powers to travel from one world to another (and I wonder why later aliens ceased from using magical vehicles?!) who continue the fight of their ancestors. On the other hand, Lost Galaxy had four humans and one member of Mirinoi (an idea ported into Boukenger's Natsuki since the series kind of pays homage to past Power Rangers in the Sentai 30 Encyclopedia but in a subtle way) where rangers from both sides are unique from each other to give distinction (only villains' names get interchanged in PR/Sentai conversations most of the time). In Lost Galaxy, there's the male to female conversion for yellow rangers for some reason.
Setting: Gingaman takes place on Earth with rangers who are descendants of people who use powers from a lost galaxy (maybe it's what inspired the title of the PR counterpart) where the forest was affected by mystical space travel (not the science fiction type). On the other hand, Lost Galaxy hardly had any reference to the forest, rather it was more on the search of another planet like Earth (hee hee, the Gingaman never went there, good imagination Judd Lynn has) so if you like Star Wars, Lost Galaxy may be for you and NOT Gingaman.
Crossovers: For some reason, Saban wanted to include the crossovers in the series' timeline (while in Sentai, fans decide whether it's canon or not), which footage from Gingaman vs. Gogo V and Gingaman vs. Megaranger crossovers. However how the crossovers occurred are kind of different due to show standards.
The enemies: I think there was a little bit of creative writing with Lost Galaxy that made it worthwhile. There was Trakeena against Captain Mutiny (Gingaman's Captain Zahab). However featuring more than one type of enemy in a single season is kind of old stuff but made fresh in different ways Power Rangers and Sentai alike. Some fans viewed Trakeena as to be more dark than Gingaman villains- I can agree. In fact, Captain Mutiny wasn't the main villain but Trakeena, which the final fate of Captain Mutiny was like in how some villains in Sentai series, killed by a fellow villain.
Deaths of heroes: The fact is Bullblack died and Magna Defender survived. Kendrix Morgan had a death scene (but she was resurrected for the finale, something that doesn't happen much in Sentai) while Saya didn't die, get replace and got resurrected again.
Overall: You'll need to see both because both have their own unique concepts. Was Judd Lynn watching too much Tokusatsu? Maybe! Most of his ideas seem to be from other Tokusatsu incorporated into Lost Galaxy than Sentai in my opinion.
By the way, I think this show is better than Time Force in my opinion, which is still a great series.