Yes, it would be nice :)

I've heard that there are other ways of getting the items but don't remember exactly how. Probably either by adding certain text to a file (guide is on Steam forums) or by a save editor.

Though it would be more fun to be able to join the community event to get them :)

There are several threads on this precise topic but the short answer is that Hello Games made the decision to exclude the unworthy GOG users and that GOG agreed with doing so and the same decision was to exclude GOG users from further participation in 3xperimental branch (beta testing of patches).

I can understand excluding a group from beta testing as having all your feedback in one place helps with finding and resolving issues. Being excluded from the Twitch Drops I don't understand as you are only rewarding part of the games players.

Just participate in the drops and link your steam account of whatever account you want. Keep track of all drops you "earn" watching those streams then use the optional ways available to claim them. A bit of a hassle but it works and cost you only a bit of time to figure it out. I agree there is no reason to exclude GOG users however.