Based on what you wrote, I couldn't possibly know
what went wrong; if you are certain you installed 1.2.20, and if
the source code for the version you installed has been updated
to actually say 1.2.20, it should be 1.2.20. I don't know where
you obtained your copy of Gorm either.
I chose to download 1.2.20 from
http://www.gnustep.org/experience/Gorm.html
myself. One quick peek into GormInfo.plist reveals that,
indeed, Gorm 1.2.20 thinks it's 1.2.18. It must be simply an
oversight during the release process. The code is definitely
newer than February 2012 (plenty of files and folders are
dated 14 Apr 2013).