Many thanks finkleroy, Cavalary, and BenKii. Glad it wasn't just me!
I asked GOG support a while ago and they said these ones were fine and could be gifted any time, though if that was incorrect I'm not sure how we'd know until someone tried to add them. I have got direct redeem URLs for each, and none mention expiry.
I think I'll give up for now, just more errors - I didn't realise it would be this complicated to give games away!
In case you have Barefoot Essentials enabled, please disable it for now.
All the bugs are gone then.
Or just follow Cavalary's advice. It's the better choice I guess.