If you need gawk, put gawk in your shebang, and if nothing else the end user has a chance at understanding why they’re seeing a crash. Also it will crash on line 1, nice and loud.
Edit: just checked and my vps than runs 20.10 uses gawk
Edit 2: 18.04 also comes with gawk, at least the install I have. I know I have worked with mawk on a linux machine- could it have been Ubuntu 16.04?
That must be what happened. I just checked my other machine that had a pretty new install of 20.04 on it, and it does have mawk. I'm happy to see I'm not crazy.
Debian Jessie (circa 2016) used gawk.
Debian Stretch (circa 2017) used mawk.
Today, Debian uses gawk.
Trying to dig in deeper makes my head explode though. mawk is marked required yet with update-alternatives priority of 5, and gawk is marked optional but with a priority 10. I guess the intention is that mawk is good enough for base installs, but you probably want gawk.
Edit: As mentioned below, awk is mawk for some Linux distros, gawk for others, and something else on still others :)