First, thank you for all this time and effort put into maintaining the giveaways, and to all the donors who "fed" them throughout the years.
Entirely understandable to no longer want to put all this effort into maintaining something intended to bring together and reward active members of a community that the higher ups of the site it's built around seem set on destroying, on the forums that those same people seem set on killing, the result of this being that the currently active community is becoming more and more just a bubble of sycophants, with ever more of the, shall we say, more discerning members leaving or being more or less explicitly driven away or silenced.
On the choice of moving to Discord, however... And you can see that it's not nearly every gamer who's on Discord, especially, just as you say, in case of those who were sticking just to GOG, or at least avoiding DRM-ed platforms. Moving to the, ahem, alternate forum would have struck me as more appropriate, but it is of course your call.
In my case, I've been staying away from any chat/messaging stuff for many years now, sticking to e-mail, this forum and the MobyGames one, ever less so but still a bit of Facebook used just as e-mail basically (no more public activity in years) and maybe a couple of other sites that also have messaging systems that can be used like e-mail, not as a chat. And in recent years I also seem to have hit a... mental limit of sites to make accounts on, so if I don't happen to already have an account elsewhere made who knows when which I may dig up to use (happens occasionally), I tend to refrain from posting anything even if I'd happen to want to unless I can do it as a guest. And I resisted using Discord despite being pointed to it all the time on for example MobyGames or by ZP people, but I did check something a couple of times on it, seeing that it can be used with a guest account. Now I gather that on this server made for this giveaway that won't be possible, which makes me wary about even having a look, even before the requiring phone number part. And if I would, seeing as I clearly won't be joining any conversations on a chat service, not sure how fair it'd be to just poke in to see whether anything I'd care about was donated and ask for it.
But, again, your call and entirely your right to make it.
... And, considering GOG's actions regarding the community, I can't in all fairness say that I'd want this giveaway to continue here.