It's possible. But we don't know for sure yet, because we haven't seen it in any leaks or rumors. We imagine Sony and Polyphony Digital will announce before the PS5 release date (2020) or at least tease something like they did with GT Sport 1st announcement trailers.
On the other hand, we haven't had any hints about a next-gen Gran Turismo game yet – and there may be one. Some rumors even suggested that GT7's release could be closer than we think (2020).