As I see gists are public repositories without license defined. So no one can use a code there except for the cases within Github itself (make fork), right? This is based on these pages:
Gists are considered like repositories with regards to licensing.
If you set your pages and repositories to be viewed publicly, you grant each User of GitHub a nonexclusive, worldwide license to use, display, and perform Your Content through the GitHub Service and to reproduce Your Content solely on GitHub as permitted through GitHub’s functionality (for example, through forking).
There’s a pretty good description of what happens when source code is published without a license in general (beyond Github’s ToS) on choosealicense.com:
Of course, if it’s about your own gists nothings is stopping you from including a license.