Okay, there's a new settings page now (visible when you're logged in) where you can control import behavior.
Default behavior for
library import changed: Previously
all games seen during library import would be removed from your GameSieve wishlist. Now games will only be removed from your GameSieve wishlist the first time they're seen during library import. (So from now on it's safe to wishlist owned games for gifting purposes and then re-import your library. And if you've never imported your library yet, you can toggle the setting to leave your wishlisted games completely untouched.)
Two non-default options are available: remove
all games from your wishlist (the previous behavior) and remove
no games from your wishlist.
Default behavior for
wishlist import remains unchanged: games present on your GOG wishlist during a previous import, but now removed, will also be removed from your GameSieve wishlist. But there's a new option to only ever add games to your GameSieve wishlist, never remove them. By using that option you can reduce your GOG wishlist in size without affecting your GameSieve wishlist, while still benefitting from import for easily adding newly wishlisted games.
Thanks for asking about this!