Is Geb’s ultimate effected by protections

A god's magical and physical protection values will reduce the amount of damage the corresponding types of damage apply. A god's inherent protections scale with level. For example, Ymir's total physical protection is 23 + 3.3 × level + Physical Protection From Items. Ignoring penetration for now, the formula is:

Actual damage = (unmitigated damage × 100)/(100 + protection value)

The result of this equation means that a protection value of 50 will reduce damage by 33%, 100 will reduce it by 50%, and 150 will reduce it by 60%.

For example (still ignoring penetration), if Ra's Celestial Beam is listed as doing 400 damage, and he hits someone with 70 magical protection, then the actual damage will be (400 × 100)/(100 + 70) = 235.

http://smite.gamepedia.com/Damage#Defense_and_Mitigation

The damage scaling for magical characters (mages and guardians) is 20% of their magical power. Physical gods get 100% scaling.

the formula is: (Base Damage + (Base Damage per lvl × God's lvl)) + (100% of Physical Power OR 20% of Magical Power)

so as an example. Anubis's base damage at lvl 1 is 35 and every level he gains 1.5. If he got 100 magical power, the formula becomes (35+(1.5*1)) + (100*.2)) = 56.5.

Anubis will deal 56 damage to the enemy if they have 0 protections.

Yes, building magical power is not only good for your skills, but your basic attacks as well.

source and more info here

I missed the part of your question about enhancing basic attacks. Yes there are a handful of items that give you an extra oomf on your basic attack. they are:

• Telkhines Ring gives extra 40 magical damage for each basic attack (so it just adds 40 to that formula above)
• Polynomicon after using an ability, you gain an extra 50% magical power damage on your next basic attack.