In the wild these gull chicks would be eating raw fish, so if you have a nearby fishmonger and can buy cheap herrings and chop them up, this would be their ideal food. However, if the parent re-discovers it, your garden will probably become a no-go zone because parent gulls are so aggressive and will attack if anybody goes near the infant. The risk is that if the parents don't find it, it may wander around your garden for ages, possibly trying to fly but also becoming dependent on you and you may get the worst of both worlds with it being a very aggressive but dependent "semi pet". Once these birds become adults and the breeding instinct takes over I suspect that any domesticity is left behind and they simply go into a defensive attack mode, with fond memories of being reared by humans left behind, so don't expect much gratitude. They're not like cats and dogs who really bond with humans.