We do our best to leave houses in the same condition that we found them in, and that includes the gutters. If I lean an aluminum ladder up against a gutter, I'm going to scratch it up.
I've never received a complaint from a homeowner saying we scratched up the gutters, and I hope I never do. My old trick was to throw a towel, jacket, or sweatshirt over the end of my ladder before leaning it against a gutter, but I've found a far better solution. Tape!
By applying electrical tape to the portion of my ladder that leans against the gutter, it no longer scratches the gutter. The tape lasts for a long time, and it's easy to replace when needed.
Shortest. Post. Ever.