zforce should force a '.gz' ending on a gzipped file which does not have that extension. However it doesn't. The offending line, line 27 in the zforce script, greps for "defl" in the output of gzip -l. gzip -l doesn't emit a line with defl in it, so the test fails and the filename is not modified.
Is fixed in gzip-1.2.4-15 - thanks for the detailed bug report.