Hi!

I recently replace an existing 30AMP 2 Pole with a 30AMP 2 Pole GFCI on my main panel. This breaker services my pool equipment - which are mostly 240 but also have some 120 lights and misc equip. I noticed that the 2 hot conductors are either #8 or #10 copper but the neutral (white) is #12. Is this normal? Is there a chance that it can overheat? Can it effect the voltage since the drop is about 100 feet? If necessary what can be done to remedy the situation other than pulling a new neutral conductor?

is it a main or branch(feeding 1 piece of equipment)breaker? GEO

I commend you for installing the GFI, but without further information about your equipment, none of us can safely make a call on this.
I'd suggest you hire a qualified, licensed electrician to check this out for you.

My friend who is a licensed electrician will be coming out this weekend to look at it. I also checked the voltages at the pool pump - and found it is 175V - while it is 240V at the Main panel. Isn't that an awful high voltage drop? Its about a 100 foot run.

It's time for a pro to look at your set-up. That is way too much voltage drop- I'm guessing that there is another problem waiting to be found. Voltage drop, under load, should not exceed 5%. A 240 v. motor should not be run at less than 204 V.
In theory, it is sometimes permissible to reduce the size of the neutral. For a 30 amp circuit, however, you are required to use a full-size (#10) neutral. Also, the use of different size wires is likely to cause GFI problems.