This may just be me, but it seems inconsistent that one of the GL sets in the set bar is "FF:GL" and the other is "GL:GF". Why is the GL first in the one and second in the other?

It's because "Fast Forces" is actually the an overarching product type, so I decided that is may be a little easier to identify them as such. So you'll probably see "FF:T" for Thor, etc. If this is too confusing, please let me know if there's a better way. It made sense to me, but I'm just one person.

I was thinking maybe the gravity feed could just be "GL" since it's the set that the Fast Forces pack is "based on." (The old Green Lantern Corps set would have to be changed to "GLC" or "GC.")

However, I wonder if the Captain America gravity feed will be numbered separately from the main set. If it is, it would need a similar type of naming convention to distinguish it from the main Captain America set as well.

Yeah; we'll have to see what they other sets looks like, and make modifications moving forward. It's harder to go back and change sets because the sets ids are locked into inventories and such.

Per this thread, it appears the gravity feed for Captain America will not have a different set symbol, and will just be numbered in the #200+ range. Do with that info what you will.