Im not a big fan of limiting unit gifting. Why shouldnt a Nation with advanced tech but low production, and nation with High commerce but backwards tech and a nation with high production but low tech be able to cooperate to arm themselves well? The game allows unit gifting, its part of the game, so I need a pretty good reason to ban or limit it.
If the reason is anything like "to stop civs from becoming too powerful" or "to make things easier on the weaker civs" then I'm not a fan of it. As I said before and I will paraphrase Ozzy again, "Good Players rise Poor Players fall" we dont need any extra rules that are designed to prevent this from happening because its just not going to work. NO MATTER WHAT rules we adopt, the better players are going to dominate. So why add rules to try and prevent this?
I will also repeat that we should avoid like the plague, rules that are designed to make human players behave more like the AI. The AI never gifts units, true, but the AI is stupid. Gifting units and cities are one of the most innovative, tactics humans use. People just call it an exploit because the AI doesnt do it so they arent as prepared for it. The AI sucks, humans are awesome, let humans be humans and innovate, and continue to surprise us.