As it stands, I can't see us willing to part with either permanently before Tuchel has had a look at least. I do think Colwill is likely to be loaned out on account of his age however - especially if we're able to get two CBs in quickly. Gallagher is a bit more tricky, we've got at least 4 guys who played a decent number of minutes in his position this season - he's not guaranteed minutes by any means and yet I think it's time he gets a chance with the first team squad if he does envision himself as a long term Chelsea player. The fees and/or buyback strategy has been employed in recent years with Tammy, Livramento, Guehi, Tomori, and pretty much all of them are having good seasons which is leading to flak for the board from the fanbase. Makes me think a high profile departure of Gallagher or Colwill(position of importance) might rub everyone the wrong way. If I was to guess - I'd say a loan for Colwill is the most likely outcome.
As for Gilmour and Ampadu, I think there's a bit of a difference in the outlook for both cases. Ampadu has had two relatively unsuccessful loan spells in a row now and his stock seems to have dipped quite heavily. He's still very young but I can't see him staying with the squad next season and I'm guessing he could be bought for the right price or for cheap with a buyback clause. Gilmour only has a year left on his contract so I reckon it might depend on what he wants for himself this season and it would probably depend on how preseason and the summer shapes up.. A lot of Chelsea fans still think he would do well given the right opportunity in the right system and want him to get more chances but Tuchel didn't seem that hot on him and that might be telling in the end.
As for other players, there's a kid called Charlie Webster who plays as a CM in the u18s and u23s and he's had a cracking season at the age groups. He's more of a creative mid from what I've seen (drives with the ball and got an eye for a pass) but I'm interested to see where he goes. If you do end up looking for a forward, then Broja would be an obvious interest I reckon.