The term "gold filled" is actually misleading - it means that the exterior of the item is gold, but the interior is some other metal. It's supposed to be a process where the gold is bonded to the other metal under high pressure, as opposed to a gold plate, which is gold deposited on a base metal via a chemical or electrochemical process.
Some gold filled items can be very beautiful and durable, at least for the near term, though I'm sure there is also gold fill that's poorly made. I don't know if jewelers would work on gold fill, however, so it would be difficult to get a gf ring sized or repaired.
Thank you for your replies! I see GF and GP stacking bands on etsy a lot. I particularly like the floral engraved ones by Deb Blazer. I wondered if a band like that would damage a gold ring if I wanted to stack them together. I figured it's probably a bad idea. Anyway, I don't have to wear gold rings with them. They're really pretty themselves and I don't have to wear them with my gold ones.
I won't buy gold plated, but I LOVE gold filled. It lasts SOOOOOO much longer than plate. I have jewelry from the 1930's that is gold filled and it looks brand new. I'm not sure about it rubbing against other metal, but by itself, it seems to last forever.
I avoid plated, vermeil, gold tone, and all that. I will buy gold filled and solid gold and that's it.