Two things:

1) Make sure your insurance carries ladder work, especially if you have a helper. If you're using a ladder to clean the gutters, make sure you're covered!

2) People that call you back next year, or 2 years from now, are going to expect that same service....even if it was 'just a special offer'. They don't forget things that are free. If someone else catches wind, you'll hear "Ya, but you cleaned the gutters for xxx, do mine too". I can't picture anyone who is looking for more business to say "NO, it was a special offer".

It's not a bad deal, just be prepared for any repeat customers to expect this next time.