[Gnome]

If you're looking for battlefield control there are many good spells for this: Glue, Grease, Force Dome/Wall, Mystic Mist (has to be cast ahead of time), Darkness, Shape Earth, Windstorm ...
Note though that none of these spells does damage! Standard GURPS Magic makes mages very weak at dealing damage but very good at pretty much everything else. In DnD, I found that wizards eventually outclassed everyone at everything. In GURPS, at least a warrior can dish out more damage (this is a warrior's specialized niche after all), even if a well-built wizard could easily defeat her in a fight with save-or-die type spells, probably has more ways to affect a broader battle between buffs, battlefield control spells, and special case utility spells, and more or less outclasses the rest of the party anyway if the player knows what she is doing...

The one exception to "mages are bad at damage" is elemental damage. If you're fighting an ice elemental, the wizard's weak fire spells are suddenly an important factor.

If you're going to modify this state of affairs, I would suggest inventing some expensive special advantages a mage can buy to improve her damage-dealing prowess or improve other aspects of magic. I have a house advantage called "Spellslinger" which effectively increases RoF and damage for missile spells. I have another one called "Huge Caster" (I reserve this for giants and such) that allows the caster to cast on large SM subjects and over large areas for cheaper. Thaumatology has lots more options for modifying Magery or the magic system to make it better/worse/different.