Try '\1' for the replacement (single quotes are important, otherwise you need to escape the \):
"foo".gsub(/(o+)/, '\1\1\1')
#=> "foooooo"
But since you only seem to be interested in the capture group, note that you can index a string with a regex:
"foo"[/oo/]
#=> "oo"
"Z_123: foobar"[/^Z_.*(?=:)/]
#=> "Z_123"