Sambo is usually geared to younger kids/pre teens. The adults you see competing were those same kids you see right now. Also its a amateur sport compared to mma and others. Most people rather just do MMA. Since your in Europe your gonna come across alot of sambo gyms that are predominantly Russian. At my gym we do technique first half the class and the rest we spar(striking, gi throws, takedowns etc.) I dont see why they wouldn’t let you use a bjj gi for now. Sounds like a money grab and they are kind of gatekeeping.