Hey!
The construction going on is to convert an existing building of the resort to DVC and is happening at the Big Pine Key building, which sits just south of the quite pool (adjacent and to the right) as you walk out of the back of the lobby. Both pools and all amenities remain open and I wouldn't let this dissuade you from staying at the Grand at all.
Disney is generally good about minimizing disturbances from construction projects at resorts and the majority (like 99%) of this work is going on inside the rooms, not the outside of the building (although it looks like they were replacing the roof shingles - but that should be done well before your September trip).
Still, your best bet is going to be the Sago Cay building, if you still want to ensure you are as far away from the construction project as possible, which would put you on the other side of the property with the quiet pool, Sugar Loaf building, and Marina between you and Big Pine Key. If you don't your preferred choice up front and end up with something facing or near the construction I have had very good luck in the past with asking the front desk for a reassign for that reason.. especially at the GF, since they always hold a not insignificant number of rooms in reserves for "walk overs" from other resorts where guests had issues with their stays for one reason or another.
Fun tidbit.. which resort does GF see the most "walk overs" from?... CSR. I guess they assume those guests are already on the cusp of considering the moderate to deluxe jump so giving them the bump has a decent chance of translating to an upsell to a deluxe category resort on their next stay.