I can see where you’re coming from buddy, but incoming TL:DR because I like me a complex character breakdown study-

okay so you asked for it

it’s true that in his first appearances, and in most of the American stories, Gladstone doesn’t get many, or any, redeeming moments: Barks himself felt that he couldn’t make the character go much further than being a rival mirror-image of Donald, and eventually sort of abandoned him in terms of character development. The super-natural luck trait itself only evolved over time, and it was really Don Rosa that ran with this idea and the possibilities it could bring to the stories. It’s when you hit the European comics, which are where many of the ‘secondary’ characters started to blossom and develop their own lore and intrigue, when things started to get interesting and the actual psychology of Gladstone’s character is explored- which is both logical, and sad.

I’m gonna throw down this copy-paste of an excellent summary of the European aspect from a post by Sarroora here;

“I prefer Gladstone’s characterization in the European comics because rather than the relationship being ‘Lucky Jerk Cousin Tormenting Poor Donald Just Because’, the relationship is bumpy because Donald himself has contributed to the tension - he’s not always a victim. Sometimes he does irrational things out of jealousy, which damages their relationship.Remember that the rest of Donald’s family aren’t jealous of Gladstone, and so it comes as no surprise that Gladstone’s relationship with all of them is better. Then there are characters like Grandma Duck whom Gladstone completely respects and loves. (Gladstone also has a thing against refusing any request from Scrooge McDuck).There is another major reason Gladstone in European canon puts up an arrogant, aloof front for everybody. He is exploited by everyone. People pretend to be all friendly and interested in his company, and when they get what they want, they take off. Yes, It’s awful and disgusting and that’s exactly how Huey, Dewey and Louie felt when he admitted to them one day during a bout of loneliness.What’s sad is that his own family do not realize that their constant requests of him to ‘give some of his luck’ make him feel objectified because of how he’s thrown aside as soon as the job’s done. When he’s the one who wants to help though, he gladly works his luck to the person’s benefit with no regrets.”

There are many stories where you can really understand where Gladstone’s arrogant and isolated character stems from this abuse of his gift, and why it would turn him away from voluntarily building connections with anyone, but I think his bitter relationship with his cousin becomes very understandable when you consider that Gladstone must know (or, at, least feels) that he will never have the type of luck that Donald has- the luck of having a caring, supportive family, and genuine friends who don’t see him as a tool. It’s doubtless worsened by Gladstone’s own conceit; when people see how he takes his luck for granted, and that he doesn’t need to try hard at anything or have anyone take care of him, then naturally nobody tries to help him. Nobody cares for Gladstone, because they think he doesn’t need them to, and because of the way he behaves, they don’t think he wants them to, either. There’s an easy mirror to this in Scrooge’s own relationship with wealth, and how he lost sight of what it really meant before Donald and his nephews made him reflect on his existence, but Gladstone hasn’t had that kind of opportunity to break out of his self inflicted isolation. He hasn’t had the long-term support to do so, and without it I suspect he’s both too afraid and too cynical to try.

TL:DR my angle is that Gladstone constantly rubs his good fortune in Donald’s face because it’s the only thing he has over him; in fact, his luck is the only thing he has. Donald might be a little envious of Gladstone, but, when you think about it, Gladstone is probably more envious of Donald, and with better reason.