So I did a post about this last year (see:
https://forums.playcontestofchampions.com/en/discussion/241499/psa-gift-two-ggcs-if-your-alliance-is-full) but I thought it was worth a reminder: if you have a reasonably full or active alliance, everyone should trade at least two GGCs (or some equivalent) with each other.
Short version: two GGCs cost 600 units (300 each). If everyone does this, you'll get back 550 units from alliance milestones and 40 units from solo milestones, which means those two GGCs will only cost you 10 units net. That's not a typo: 10 units. Meanwhile, on top of the two GGCs you'll get back in trade, you'll get a ton of solo and alliance milestone rewards. All for basically nothing.
While an honest debate is happening about the benefits of the Gifting Event (and who can participate in it), I would recommend not leaving rewards on the table out of spite or confusion. This is the rare time when there are a lot of rewards being basically given away to anyone who can and is willing to participate, and understands the numbers. If you don't want to spend, don't spend. But at least take the free stuff that's up for grabs, and make sure the rest of your alliance mates get in on the easy rewards.