Using gawk-only features means sacrificing portability, but gawk is so much more refined than the other awks. I don't know why it's not the default awk on a lot of OSes.
wouldn't installing gawk on all target machines take care of portability?
Portability usually means taking the machine as you find it. If you're pre-installing stuff then that is just configuring your environment.
Yes. As long as you have the right version of gawk, you're good to go.