I don't see this answer anywhere, so I figured I'd ask here.

What happens when you're subscribed to pay monthly and someone gives you their buddy gold?

Gold Accounts are considered just one perk of the subscription, and have a countdown of their own independently from subbing. If you click on Personal > Dashboard you’ll see at the bottom a section with Gold Account Info. I believe—I might be slightly off but—I do believe the part that says “Time Left (roughly)” shows how much Gold Account time will be left after the subscription ends. So if you have a lapse in or cancel your subscription, you’ll still have the Gold Account for however long the timer says. Every time someone gifts you a buddy account the timer should increase by a month.

that makes perfect sense, actually! Thanks!

You can actually opt out of being given a Buddy GA, if you go to Dashboard and then Preferences, but it's not very well known. I opted out because people kept on giving me Buddy GAs that weren't needed because of this subscription, and I know that not everyone wants to be given one (unless they somehow change it so that it can be activated at any time like the medals rather than right away).