Forget Gateway address. That only pertains to routing network traffic from one network to another. ArtNet doesn't have anything to do with that.

Simon is right about tightening the subnet though but with only a few devices, it won't make a difference. If you wanted to you could even go as tight as 255.255.255.252 which is the subnet for only two addresses x.x.x.1 and x.x.x.2 but this is still not necessary.

255.255.255.0 is fine because it works for all the addresses in the last subnet. x.x.x.1 - x.x.x.254.

A question... Is there a reason you're using ArtNet 0 universe 1-3? Could you try something higher than 4 for the universe or try switching the Artnet subnet to something higher than 0? The reason I ask is because Catalyst has a default of broadcasting to the first 4 universes so making a change from this helps alleviate problems.