VI
In fairness, they
might already have been working on VI for a while now. I don't have any proof for that whatsoever, however, I do know that almost immediately after the release of GTA V on the PS3 they were quickly working on RDR2 as far back as 2013/2014.
Yes, I do think that they're milking GTA Online for all its worth, but the next game is probably going to be significantly bigger than GTA 5 in scope, bringing
Vice City, Liberty City and Los Santos cities/maps into one game, which is very ambitious, but very time-consuming and requires almost every studio to be involved. Again this is just from credible leaks, but if they're true, then that partly explains the extended time frame between V and VI.