Usually, the full Gettext package is not needed at runtime.

For example, glib requires using the gettext() function provided by libintl or the system.

Quoting the "PACKAGING" file in Gettext:

Packaging hints for binary package distributors

Although the source of the gettext package comes as a single package,
I recommend that in distributions of binary packages the installed files
be split into three packages:

gettext-runtime
Contents: Runtime libraries and programs.
Audience: Anyone who wants to run internationalized programs.

libtextstyle
Contents: Text styling library.
Audience: Anyone who wants to run or develop programs that produce
styled text, to be displayed in a terminal emulator.

gettext-tools
Contents: Tools and documentation for developers and translators.
Audience: Anyone who wants to develop or localize internationalized
programs, i.e. developers and translators.
Dependencies: requires gettext-runtime.

The 'gettext-runtime' binary package is much smaller than the 'gettext-tools'
binary package (420 KB versus 5.5 MB). I recommend to include it in the default
installation list, like GNU 'coreutils'; in fact, the runtime programs are
utilities for sh programs.

Gettext_jll.jl currently only provides: LibraryProduct: libgettext

This is part of gettext-tools.

Can I create a new package GettextRuntime_jll.jl with only the runtime?

The runtime includes lib/libintl, lib/libasprintf, and a few executables listed in "PACKAGING" file in Gettext.

xref: #792