Gawk is part of "base" pkg and expected to be installed on every system.

gawk:
groups : base-devel

$ pacman -Qi base | grep 'Depends On'
Depends On : filesystem gcc-libs glibc bash coreutils file findutils *gawk* grep procps-ng sed tar gettext pciutils psmisc shadow util-linux bzip2 gzip xz licenses pacman systemd systemd-sysvcompat iputils iproute2

Emphasis mine.

IMO its not about being in base-devel or base being mandatory (besides that base may in theory change at any point in time). Transitive dependency omitting is not a good technical practice for important runtime dependencies. The declaration is required in order to allow pacman actually do appropriate build order calculation based on a graph which is especially important for initial bootstrapping installation. Omitting first level runtime dependencies, especially such that may be used in install scriptlets may lead to all kind of funky issues. Taking this into account, I'm always in favor of first level dependency declaration and hence will add gawk as being essential for pacman-key.

This was fixed in 6.0.1-7 via  FS#75377 . (The latter is an unfortunate duplicate, which I opened assuming the problem would belong to FS project pacman.)

Btw. that other report states a tiny example why, as Levente already pointed out, coverage of awk by base[-devel] isn't necessarily a fix for the problem.