Gai is very similar to Shinkengold, Genta in many ways. First there is the fact they are comedy relief characters and that they show much more power than we think. They are completely hyper characters as Genta wants to be a shinkenger and respects Takeru very much while Gai wants to be a Gokaiger and respects all super sentai, expect Doc though.
I love both of these characters a lot though, Genta was a great and funny character, but also had many more traits that are enjoyable like how smart he is. Gai is funny and i loved his very long role call and it seems he can catch on to things easily. Now both of these teammates werent offically part of the team with Genta, Takeru wouldn't expect him at first. I can this will happen to Gai as well since he is human, he probably has to do something that would impress Marvelous, which will probably be his mech or that he has three sentai powers with him.
In the next episode he will have to prove himself and he will show that in a fight with the team at a familiar place. What is also odd is that Genta was accpeted part of the team in episode 18 like Gai will be also their first fight with the team will be at the same place, odd isnt it.