Seems you have asked a similar question before...with an A99 nearby.
I have an opinion regarding your question, but I can't tell anything about BW from modeling. I can't model the GM to show a good match, and I also can't model the choke at the bottom that makes it work so well.
Personally, I think the GM design really depends on being high and well into the clear. At my location both mounts are 36' feet apart and at least that far away from other stuff close by, trees and wires.
Here is what a center fed 22.5' foot radiator shows me with a Starduster and a A99 36' feet nearby at similar heights:
Here is the same GM with both the other antennas removed.
There's not much difference and that is probably why I don't see much affect. If you're expecting a beam like pattern to be produced, I don't think that will happen either. It may produce some kind of lopsided pattern however.
If your idea is to place the other radiators closer like a beam, then maybe you'll see some directional effect in the pattern. I'll have to check that out with the model I made for you before.
I don't think your idea about BW will be of benefit. Like a 1/2 wave dipole or an end fed 1/2 wave has a nice wide bandwidth standing alone, when they are placed close like a yagi beam, that BW may pretty much go away. So yes, IMO you'll loose the BW.