For any change of more than just a few lines, you will need to assign copyright in (that is, ownership of) those changes to the Free Software Foundation.
This is generally an easy thing to do. In particular, you can choose to
use a version of the copyright assignment which assigns all your current
and future changes to gawk to the FSF. This means
that you only need to do the paperwork once, and from then on all your
changes will automatically belong to the FSF. The maintainer recommends
doing this.
The maintainer will help you with this process once you have a contribution that warrants it.