Can a floor levelling compound be poured over a gas pipe in a concrete floor?

It depends on if the pipe is copper, steel or yellow plastic. If anything other than plastic polypropylene, no concrete including Latex self levelling can be used as the two react and the cement eats into metal, copper risking leakage over years. All metal/copper pipes must be fully wrapped in a product known as Denso Tape if there is risk of this pipe touching concrete (including Latex). Insulation does offer similar protection from corrosion but this is not good practice as this can allow cement contact as Pipes are hard to fully wrap correctly. Normally gas is set into a vented duct to allow gas escape and detection. Check via Building Inspector as part of your build requirement to comply.

Yes you can as long as the pipe has been wrapped in tape or in a pvc sleeve. Any cement product will reacted with metal, copper etc