I don't think a player swap is likely, perhaps someone on loan but anyone under contract at Chelsea has a right to play there and would have to agree a move to Everton. That being said, they can play hardball with their players "you won't play here", etc, but anything beyond a cash sale for gordon is probably too complicated.
As for 50m pounds for Gordon, I'd be for it had we not sold our best player already this summer and weren't short on attacking prowess right now. Replacing Richarlison is tough enough, I worry how we'd look without Gordon.