It depends. The line between MPR and HPR can be a bit complicated at times. If the rocket contains more than 125g of propellant, or weighs over 1500g, it is an FAA Class 2 rocket, which requires a waiver to be flown, and would also require a HPR certification to be flown at a NAR or TRA sanctioned event (or in any location where NFPA 1122 and 1127 are law). Also, if the rocket contains a single motor with over 160 N*sec of total impulse (H class or larger), or a combined total impulse across all the motors of over 320 N*sec, it is considered HPR by NAR, TRA, and the NFPA (so again, would require a cert to fly at a NAR or TRA sanctioned event, or anywhere NFPA 1122 and 1127 are law). Finally, if the motors (individually) have more than 80N of average thrust, or use a 'sparky' propellant, they are considered to be HPR by NAR/TRA and NFPA (so the same restrictions apply).
Your example of 6 G motors would definitely have more than 320 N*sec of total impulse, so it would require a HPR cert to fly at a NAR or TRA sanctioned event, and anywhere that NFPA 1122 and 1127 are law. Further, it would certainly have more than 125g of propellant (the lightest G motor I could find on thrustcurve has 39g of propellant, so 6 of them would have over 200g of propellant), and as such would be a Class 2 rocket requiring an FAA waiver to be flown.
However, a cluster of 3 of those same G motors (It's the Aerotech G74) would only have 117 grams of propellant and 248 N*sec of total impulse (and doesn't use a sparky propellant), so as long as the rocket weighed less than 1500g, it would be considered low power.
