What would make you think it swallowed something?
If the mouth is stuck open? If something is in the mouth you can see. You would be able to hold the fish and remove a pebble. More info if you think it is.
But there shouldn't be mouth sized gravel, GF are always sucking stuf off stones and spitting them out. I had a GF in my pond have a stuck pebble and survived to live on after the pebble was removed. It was introduced by a water lily with gravel on top of the soil in the basket.
It could be a water quality issue.
New to FW fish? First tank in a long time or EVER??? Need some info on keeping fish?
Start here - Aquarium Advice Article
FW Favorites right now Clown Loaches, any shrimp - SW Vase Red Mangroves