At the end of "Pound of the Baskerville's", McDuff approaches the Rescue Rangers with some papers in his mouth. Gadget says "Oh, he's got something for Chip!" even though McDuff never said anything. How'd she know?
Gadget is the brains (what? you thought Dale was?), as well as the beauty, of the team, so it was elementary for her to rightly deduce that the hound had the lost manuscript.