is this color basically "gunship gray"?

Not sure if it will be gunship gray.

If Revell recommends a colour I am 100% sure that it will be from their own colour range.

The makers that have their own colour ranges will ALWAYS refer to them rather then the generic colour ranges like Humbrol or Gunze Sangyo.
If they don't have the actual colour they will give you the ratios to mix them even though another colour range/company might carry it.

HEY,
If you knew what kind of blue this was, you could find the nearest color blue and use that to mix it with the gray. Or you can cheat like i woiul and use a different color than the recommended mix.

Randy