Bottas looked good at Williams when he had Massa as his team mate. Up against Hamilton he's been mostly all at sea, which actually shows how good Nico Rosberg was in some ways.
Gasly, I'm not so sure about, he's not got to grips with the Red Bull that's for certain, would he do better elsewhere? Maybe. After all HH Frentzen struggled at Williams in 1998 as I recall, then he went to Jordan and things improved for him. Some teams and some cars just don't suit some drivers, but these days they often fail in one car and it's the end of their career sadly.