I'm gonna assume you have a car with decent paint, are not ready/budgeted to do a repaint.
I've found that old, dry seam sealer in the roof gutter strips out very easily with a 2" wire brush in a drill.
You'll not want it to dig in and run across your roof, though.
I'd put a layer of blue painter tape down, then 2-3 layers of regular masking tape and then some duct tape. Make it 3-4" wide, too. Don't do it in the sun, you want to minimize the tape from pulling any paint off if your paint is getting thin.
You might need to remove any stainless trim on the gutter, too. A small screwdriver or icepick can get the areas in the corners that the wheel can't get to.
I'd then put a thick coat of Rustoleum-type paint in there (hopefully you can find a suitable color) and then keep an eye on it.
I did this very thing to a 65 Fury about 15 years ago. It sat outside for about 5 years under a tarp, and to this day there are no visible issues from having paint in the gutter ILO seam sealer.