I guess it's all due to budgetary reaons.
They have barely shown Grodd during this season, always in the dark or with "movie tricks", just for seconds until this last episode where they also had him mostly in the dark.
Being hidden in the sewers made sense for the plot but was probably a budgetary choice because with a tv budget (specially if it's not for really big productions like the ones from the HBO, never seen in The CW) you just can't have the kind of fx that are seen in big feature films, creating a somewhat realistic big cgi gorilla with that kind of budget must be really hard, and it looks much better in the dark, it would probably look much worse if he was shown in the light of the day, and they couldn't barely have him in a few scenes in this episode, they probably just couldn't afford to have him as the main villain for a full season, like they probably couldn't afford to have him in too many scenes and all of them would probably set during the night or in dark places, the viewers would eventually notice it and would wonder why Grodd is barely shown in every episode and always in the dark if he is the main villain, it worked in this season because he wasn't the main villain and he didn't have to appear in several crucial scenes of every episode.
I might be wrong, maybe he'll be seen more usually during season 2, but if it was as the main villain they would probably need a much bigger budget or cut the budget for other stuff. I think that's the main problem, decent cgi costs a lot of money, and I guess it's harder for a relatively cheap tv show to pull this kind of stuff in every episode, making a cgi Flash in certain scenes is probably much easier, if they didn't do a really good job Grodd could have looked really ridiculous in this episode, they can't just call Andy Serkis and ILM to make a realistic big gorilla for every episode.