That flight at the moment is a refurbished 3 class 777 so it will have newer seats and upgraded IFE.
A look on ExpertFlyer shows row 42, 43 and 44 JK free so personally I'd choose 43. They're pairs so no one next to you but you'll be last off the plane. However if your quick once off and walk briskly it's sometimes amazing how many other people you can waltz by. IIRC exit seats (row 32) open from 14 days out for payment but I'm not 100% sure on that to be honest.
Row 29 seems to be a standard seat but on the old mid J 747s 29A and K have extra room so maybe that's what your thinking of perhaps? If so that's not applicable as no BA 747s fly from Gatwick.
For info here are the details from EF at the moment.
BA 2273
LGW - 05/11/19 4:55 PM
JFK - 05/11/19 7:55 PM
777
Daily
69% / 30m
J9 C9 D9 R9 I8 W9 E9 T9 Y9 B9 H9 K9
M9 L9 V9 S9 N9 Q9 O9 G9
Enjoy your trip and first visit to the US of A!
Pete