Yes, having control over the cal is VERY important, but making the hardware work together is also important. One thing to keep in mind is that the compression ratio is going to be reduced by using the g-head. This combined with the fact that there is NO quench or swirl and yeah, you've got a total turd down low. Now, if you were to shave the head (cam timing MUST be corrected if this is done enough to make a significant change in compression ratio), or get custom pistons (no more expensive than off-the-shelf, they just take longer to get), or a combination of the two, now you're going somewhere. Of course the cal is going to be completely different for a set-up like this.
Other things to consider: what turbo, where does it spool, what FD ratio, what transfer gear ratio (auto only), what is the TC stall (auto only), cooling (I highly recommend installing an oil cooler, and the 4th-cylinder cooling mod might also help in this case), what cam, cam timing, what exhaust size.
Now, so far I've inferred that the head is stock (except being shaved if you were to do that). However, a good port job and chamber work can make a pretty big difference. If you don't know how, or don't have a lot of experience doing this work, especially with our heads, this is best left to someone that does know. Really, if you want a good head without a lot of trial and error, suck up your pride and pay for somebody to do it. It's not cheap (usually), but in a lot of cases you get what you pay for in this department.
Anyway, my overall point is that there are a LOT of dials to turn to make a combination work. Get it right and you'll love the results. Get it wrong and you'll want to burn the thing to the ground.