Considering the millions that GTA: Online has been pulling in, I don't think we'll be seeing a new GTA title for sometime. But also, another one is definitely on it's way...I mean, GTA is their biggest series and has been by far the most profitable and popular, even more so with the advent of GTA: Online.. I wouldn't care if another GTA took another 5 years to get released,...they're still releasing content for GTA: Online that I'm fully enjoying on PC....Soonest that I"d guess that the next GTA might be released, is 2020/2021. It's also not their only title they work on, Red Dead 2 is coming out soon, and they've got more people working on that this year, which means less people working on the next GTA and less working on current GTA Content.
TL:DR It'll happen, just not any time soen.