i usually gamble for bone helms, bone shields, rings, and amulets
amulets you really want at least cLvl 90 but 95+ is ideal to get +2 skills
rings i have mixed emotions over. i never seem to gamble gg rings, but it's a good gold sink. probably just too many mods possible
bone helms you can get 40+ life at cLvl 80-something. helms don't have a huge number of possible mods and they are most valued helm type you can gamble
shields are another mostly gold sink. can get good rolls at lower levels than amulets and bhelms but there are a lot more possible mods to spawn
you can also get good belts (ignore sashes and light belts), boots, gloves. i'd advise ignoring gantlets and greaves due to str reqs reducing their usability and therefore value
remember gamble iLvl result = cLvl-5/+4
so a level 90 char gambles iLvls 85-94. amulets require iLvl 90 to spawn +2 skill. so level 90 gives you a 50% to be high enough lvl to spawn +2
char level 95 gambles iLvl 90-99 guaranteeing any amulet will be high enough to possibly spawn +2
good luck gambling^^