Stuff I think are in character / could be canon for Gladstone:
He has tried astral projecting (because of his interest in paranormal things lol. Imagine him appearing in Donald's dreams just to bother him in his sleep too😂😭)
Could try doing a mukbang youtube channel (or whatever is its Duckburg equivalent) to see what's the hype. And because it's literally being paid to eat, barely any effort from his side XD
Since he's a big sweet tooth, he probably likes his coffee sweet as well, maybe with cream and fancy stuff too idk
Has undiagnosed (or diagnosed with Ludwig's help) narcissistic personality disorder and in this essay I will-
I think he has no allergies, considering his luck. Or he could be allergic to an oddly specific thing for a day just for it to cause something lucky to happen again lol
He would like or at least has tried sushi once. I still remember how he swallowed those sardines in DT17 raw hahaha XD And he likes shrimp and caviar?? Come on, give that 🦆 his raw seafood treat! (Would his 🪿 side enjoy some seaweed salad too?)
Feel free to add more if you have any headcanons/theories/'could-be-canon's!