I am still missing unique leather gloves in the chronicle.

According to the database they are:
Req level: 5
Quality level: 7
Treasure class: 3

So does anyone know, what level I need to gamble them?

try act 1 normal, maybe cows normal

Lvl 11-12 for gambling but you could just MF normal super uniques like blood raven, rakanishu and andariel in normal they have good chance to drop due to small drop table when it comes to uniques.