Geany-Plugins package should ship `gspawn-win32-helper.exe` and `gspawn-win32-helper-console.exe` that is needed by GSpawn, which this plugin uses.
As a workaround, one can install the [msys2](https://www.msys2.org/) package `mingw-w64-i686-glib2` and copy the above executables from `C:\msys64\mingw32\bin` or even simpler, from the official builds like [this zip file](https://ftp.gnome.org/pub/gnome/binaries/win32/glib/2.28/glib_2.28.8-1_win32...) in the `bin` directory to Geany's `bin` directory found by default at `C:\Program Files (x86)\Geany\bin`.
Probably the best long-term solution is for the plugin to use the `spawn.c` functions like Geany itself does.