Would the GPP phase of weight training be when one would be in a hypertrophy phase?
Only if hypertrophy is necessary. As a quartermiler, I would guess that it shouldn't be a primary concern.
And would it be OK do be performing olympic lifts this early? or would it be best to stick to circuit and hypertrophy type work?
I think you'd be best to devote a large portion of your training to the development of speed and power enhancement using general rather than specific means. So yes, Olympic lifts could and probably should be incorporated into your training program.
Moderate weight, reps and volume?
It depends what you're trying to accomplish.