Here's the current plan for the magic system. Note that the conditions changes are pretty relevant here.
All casters are dependent on two different ability scores, as determined by their class. Their base casting ability affects magical stamina they have-- how much magic their bodies can store, and how how fast they regain magic. Their advanced casting ability affects magical skill-- how complex a spell they can cast, and how hard their spells are to resist.
Casting a spell is a standard action which provokes attacks of opportunity. To cast a spell, you must have an advanced casting score equal to the spell level -2. Unless a spell description specifically mentions otherwise, you may only cast one spell per round.
(As 3.5, except that the skill roll to identify your opponent's spell is an Occult roll, and you may use any spell of the same or higher level with the same descriptors as a counterspell-- so, for example, you could use a call lightning spell to counterspell a lightning bolt spell.
Is impossible. Barring feats or class features, you always provoke attacks of opportunity for casting spells.
All casters have a certain number of spell points, as determined by their class(2). Casting a spell requires the expenditure of a number of spell points equal to the level of the spell being cast. Zero-level spells, also known as cantrips (for arcane magic) or orisons (for divine magic) cost no spell points, and may be cast at-will.
If a character has multiple spellcasting classes, add together all spell points gained from arcane classes into a single arcane pool, and all spell points gained from divine classes into a single divine pool. A character with multiple arcane casting classes can use spell points received from any arcane casting class to cast spells gained from any arcane casting class, although class level still limits how high level a spell he can cast. Multiple divine casting classes work the same way. However, you may not use spell points from arcane casting classes to cast divine spells, and vice-versa.
Characters gain bonus spell points for having a high base casting ability. Your bonus spell points are equal to one-half your base casting ability multiplied by the highest level spell you can cast. You may only gain these bonus spell points once for each pool, arcane and divine.
Spell points regenerate over time. A character regenerates a number of spell points per hour equal to his base casting ability. While sleeping, they instead recover a number of points per hour equal to their caster level plus their base casting ability.
The saving throw DC against a spell or spell-like ability is 10 + twice the spell's level + the caster's advanced casting ability.(3)
Some creatures have spell resistance-- a natural or unnatural resistance to magic. Creatures with spell resistance gain a bonus on saving throws and opposed checks against spells and spell-like abilities equal to their spell resistance(4).
In the absence of specific feats or class features, no more than one metamagic feat may be applied to a given casting of a spell.
By default, casting a spell takes 1 full round (from the beginning of one turn to the beginning of your next turn). If you are damaged while casting, you must make a Concentration check with a DC equal to the damage taken or lose the spell. You make your caster level check at the beginning of the casting time. If you fail, you only waste a single standard action. You do not expend spell points until the spell is finished casting.
Becoming magically focused is a move action that provokes attacks of opportunity. While magically focused, casting is only a standard action, but your speed is reduced by 10 feet and you take a -5 penalty to skill checks due to the effort of maintaining focus. Any time you are damaged while magically focused, you must make a Concentration check with a DC equal to one-half the damage taken or lose your magical focus.
Allows you to move up to one-half your speed when targeted by a spell allowing a reflex save, at the cost of your next move action. Improved evasion either allows you to move your full speed, or move half your speed without having to give up your next move action.
(1)- In G&G, modifiers and ability scores have been merged. A 3.5 score of 14, say, translates to a G&G score of 2.
(2)- As a general rule for conversions, add up the total number of spell levels a character receives per day. Thus, a 3rd level wizard gets 4 spell points (disregarding bonus spell points)- 2 from his 2 first-level spells, and 2 from his 1 second-level spell.
(3)- Saving throws will have the same good/average/poor progression as base attack bonus and skills, so a 3rd level rogue's reflex save might be +6, as opposed to +3 in 3.5.
(4)- As a rough conversion, G&G SR = 3.5 SR divided by 4