Don't use silicone. It will eventually fail and you'll be right back leaking again
If there is enough room to get a small piece of tape down into the gutter, I would suggest using a piece of aluminum duct tape...the type of tape used to seal duct work togather at the seams. Very thin, but very sticky, and will hold up under water. If you can get a small piece down into the gutter to bridge the gap, it will solve the leak problem.
If no room to use the tape, use some rubberized roof flashing caulk/adhesive. It's black, will not "harden" (stays flexible), and will seal the gap. Use a small icecream stick to apply it into the gutter and spread it. A caulk tube runs a few 3-4 bucks at Lowes/Home Depot.
Steve & Cheryl + Charlie & Casey, our furry kids.
2012 Forrest River Lexington 283ts
Toad, 2013 Subaru XV Crosstrek
Camping trips this year so far...8