Gambling any item gives item level of character -5 to +4. So +2 can appear when gambled from level 86 upwards
For amulets to always have a chance of +2 skills, you need to be level 95.
Circlets get a +3 bonus to the affixes, so need to be 92.
Coronets get +8 need to be level 87.
Tiara +13
Diadem +18.(but because of the item level a diadem will always have affix level 99 so this is a moot point)
You can of course never know if you’ll get a tiara or diadem when gambling coronets.