gets will use Kernel#gets, which first tries to read the contents of files passed in through ARGV. If there are no files in ARGV, it will use standard input instead (at which point it’s the same as STDIN.gets.
Note: As echristopherson pointed out, Kernel#gets will actually fall back to $stdin, not STDIN. However, unless you assign $stdin to a different input stream, it will be identical to STDIN by default.
http://www.ruby-doc.org/core-1.9.3/Kernel.html#method-i-gets