You have several choices for Boeing Gray. Here's what I've gleaned from Airlinercafe.com:
Xtracolor X363 Boeing Gray
DACO Airliner Wing Grey (PA40)
Mr. Hobby/Mr. Color Light Gray #338 (exact match to sample from USAir paint shop)
Tamiya Light Gray Primer (very close to Mr Color #338)
Model Master gloss white (6 to 8 parts) and Testor's 1138 gray (small bottle)(1 part)
If you can find a bottle, Model Master Canadian Voodoo Gray was a good match, but they discontinued it. I haven't used any of the above recommendations, other than Xtracolor and Can. Voodoo Gray, since I stocked up on both a while back and still haven't used up my supply, yet.
For Corogard, Xtracolor and I think Daco make it right out of the bottle. It's basically a clear coat with aluminum powder mixed in and can be anything from a light metallic gray to a darker metallic gray. I've seen people recommend mixes such as Metalizer Aluminum with a few drops of black, or Metalizer Steel straight out of the bottle. Depending on the model you're building, you can often even find Corogard decals, which save you the trouble of masking.
HTH.
Ben