Whilst playing a Tag Duel, Sky Galloping Gaia could not be Fusion Summoned by using Gaia the Fierce Knight in hand and Baby Dragon as material.
Bug reproduction steps
Normal Summon Baby Dragon with Gaia the Fierce Knight and Polymerization in hand.
Although it is not pictured, the monsters on the opponents field are W-Wing Catapult, Humanoid Slime, and Road Synchron. There is no floodgate or lingering effect that would alter this interaction. The card is currently Set on the field with the cursor hovering over it.
Baby Dragon and Gaia the Fierce Knight should be eligible materials for Sky Galloping Gaia the Dragon Champion.
What OS are you using