1) Guardian's Amulet of Life Everlasting
2) Guardian's Circlet/Tiara/Diadem of Life Everlasting

Guardian's +3 to Defensive Auras Skills
Affix level: 60

of Life Everlasting Damage Reduced by 10 to 25
Affix level: 45

What is the best clvl to gamble each item?

"- Only your character levelmatters. the item lvl is dependant on your character lvl. The item lvl will be -5
to +4 your character lvl. This means that if you are lvl 90, the item lvl will be varying between 85 and 94.
Please note that there is a difference btween item lvl and required lvl.
The mods possible to spawn on the item depends on the item lvl. An item lvl cannot be seen in the game."

That means to get the smallest pool of possiblities the best clvl would be clvl 56?

For amulet yes, for coronet, tiara, diadem even less due to their inherent mlvl bonus.

Are you sure?

From Arreat Summit:

Guardian's +3 Paladin Defensive Auras Amulets(60), Circlets(60), Paladin Shields(60), Shields(60)
Life Everlasting Damage Reduced by 10-25 Amulet(45), Circlets(45)

the affix level doesn't change but the inherent bonus on those items allows them to get the desired affix lvl at a lower clvl