He is on the cover because one of the short stories is about Gazlowe, and because there is a goblin presence. He is not a major character in any way. He is as important as Khadgar was in Dragonflight, maybe even less so because Blizzard literally did not talk about Gazlowe in any way whatsoever. If you didn't follow the nitty-gritty Beta storyline, you wouldn't even know he was in the expansion. Hell, even if you do, I wouldn't blame you for missing it because he is irrelevant in the actual expansion as well.
Wouldn't be the first time they had promotional material about characters that weren't really relevant and just did it to flesh things out.