Hi, I'm pretty new there killifish and very new to this forum. I wanted to know if keeping a pair of a. Gabunense in a 20 gallon community tank with Cardinal tetras and chili rasboras was doable?

I would say it's totally doable, especially in a 20 gallon tank.  If you want to breed them, it might be less effective compared to keeping  them in a 5 gallon tank by themselves.  I'd try it and put a hanging mop in there, the gabunense would likely be the only fish interested in spawning in the mop.  The other way you could try it is to have the tank heavily planted with floating plants and the fry may show up.  Good luck and let us know how it works out.