Welcome Wade!
Definitely take out the gravel. Definitely definitely. You can't be watching them all the time and they have a tendency to snuffle around for food. They open their mouths very quickly to suck in food, and often take in gravel with it. You might even think they have spat it out, but you would be surprised.
I originally had mine on gravel as the pet shop told me to - once I found this forum I realised how dangerous it was and swapped to a thin layer of sand. I kept finding gravel pooped out for about 4 months afterwards.
They can eat stones up to the size of their heads - and we've seen many axolotls die or become seriously ill and require hundreds of pounds worth of vets bills to recover.
So - even though you may not have seen it happen it is very likely they have gravel inside. This can cause them to float as they try to pass the stone through their system.
If the floating persists to the point your axie can't stay on the bottom, you may need to consider putting her in a shallow container so she doesn't get stressed. At that point she may need to stay in the fridge to help her pass it.
That aside, there ARE other reasons axolotls float - sometimes they just feel like it, or want to be near the top if the oxygen levels are low in the tank. Try keeping the tank as cool as poss and maybe running an airstone very gently, that might help!
But get rid of the gravel