Enter this in the search box:

Find cat gists with greater than 100 stars.
cat stars:>100

Get all gists from the user santisbon.
user:santisbon

Include anonymous gists in your search for cat-related gists.
cat anon:true

Excludes all results containing cat.
NOT cat

Search all forked gists for results containing cat.
cat fork:only

Find all gists with a ".bashrc" file.
filename:.bashrc

Find all cat gists with HTML files.
cat language:html

Find all instances of join in gists with a coffee extension.
join extension:coffee

Find all instances of system in gists containing a file larger than 1000kbs.
system size:>1000

Full code search: https://help.github.com/articles/searching-code/