Even though this is not an official source, Tibia-Stats Spell Damage Calculator is a very reliable tool because it was built using a lot of data and testing. The interesting thing there is that you can calculate rune/spell damage based on your level and magic level. There's not that much variance, though. If we take an example of a lvl 500 ml 130 mage, we get the following values:
GFB: min 263, max 487, avg 375
Fireball: min 346, max 507, avg 427
The results were the same if using the equivalent Ice runes.
So, if you are interested in dealing maximum damage to a single creature, using single target runes (Fireball and Icicle) yields better results than AoE runs (GFB and Avalanche).