The pattern for all files under all directories is usually ./src/less/**/*.* or ./src/less/**/*, either should work.
Generally speaking, it’s better to match specific files extensions — even if they should all be the same — to prevent grabbing system files or other junk. In that case, you can do ./src/less/**/*.less for just .less files, or something like .src/less/**/*.{less,css} for both .less and .css files.
The Grunt website has a pretty good reference for the majority of minimatch globs. (Both Grunt and gulp use minimatch, since it’s the glob library for pretty much everything Node related.)
It would be nice for gulp or minimatch to have their own complete docs, but that’s open source for you.