Can the gravity bow spell be mixed with magic bow that has +1 fire on it? Or can you not have both magic effects on it together?
Yes, You Get Gravity Bow's Extra Damage When Using a Magic Bow
If the character is wielding a magic bow when he casts gravity bow, he'll do more damage from the virtually bigger arrows and more damage from the enhancement bonuses and weapon special abilities of his bow.
The increased damage from the spell is an effect of, essentially, bigger arrows: "Any arrow fired from a bow or crossbow you are carrying when the spell is cast deals damage as if one size larger than it actually is." This extra damage isn't an enhancement bonus--or any other kind of bonus, for that matter--so stacking bonuses isn't an issue.
They add just fine. These are independent magical effects.
When combining magical effects, you mostly need to worry about stacking (see here), but that does not come into play here.