Within Vallejo's line of Model Air paints, what would be the closest choice to represent Gunship Grey?

Trying to find a Vallejo Model Air color to match FS36118 is a subjective call. you can take a look at this page at Cybermodeler which lists approximate equivalents:

F-15E Strike Eagle Camouflage

Then, there's Scale Model Paint Finder which sez that Vallejo - Model Air 71.053 Dark Seagreen is a close equivalent.

Since you didn't indicate the scale of the kit you intend to paint in FS36118, the Cybermodeler link provides scale fading equivalents.

Model Master Acryl Gunship Grey is a good choice. But, that's just a personal opinion.