It seems the wingspan of the -8 is a bit bigger than the -400, and so is a category F plane rather than E, which seems to limit the stands that can be used. It the stand is too small, GSX will not give you ground services.

Gates listed as HEAVY in the starting location dropdown in P3D aren't always big enough. Navigraph parking charts don't list the aircraft supported, as far as I can see. (Although there are special taxi charts for cat F aircraft).

So how can you find suitable stand numbers?

(I've just googled for stands the a380 uses which seem to work ok for GSK - KSFO: G101 EGKK: 110)

 

For GSX, it’s a matter of setting the wingspan in the position customization screen. Once that’s done, you can use the GSX YouControl menu or whatever it’s called to find suitable parking spots.

The easiest way to find the right spot is to spawn on a runway then use GSX to warp to a compatible spot. 

Alex Pugh

Navigraph has some parking stand charts for the 748 or type F aircraft in general (for example EGLL 10-9C2, EDDF 10-9C3). In some cases the 748 taxi map will show the available stands (for example KMIA and KLAX). If those charts aren't available, I'd probably use Flightstats or Flightradar24 to lookup the real world gate used. Of course, you can always change the maximum allowed wingspan for a stand in GSX (page 30 and 34 of the manual).