On Debian 10, I followed the instructions to build gpg4win but got an error when running:
$ ./autogen.sh --build-w32 ... configure: convert is missing configure: error: *** *** Required tools not found. Please consult the above messages *** and install them before running configure again. ***
Prior to this step, I installed all the packages listed under Basic Requirements. However, it appears that imagemagick (which contains convert) was not included.
Adding imagemagick to the build requirements resolved the issue.
Fix available here: https://github.com/drichardson/gpg4win/commits/fix-convert-missing