Granules are embedded in the asphalt, not “glued.” The only true way to embed granules in a shingle is to possibly heat the shingle to a near-melting point and embed new granules. I DO NOT recommend doing that- this was merely to prove a point.
If the granules are missing, you can expect degregation of the asphalt to the fiberglass mat much faster than the remaining heathy shingles. If this was my own roof and these are fairly significant spots, I would have them replaced.
If you can, disconnect the bottom section of the downspout and work with a shop vac. May be your best hope at an easy fix.