Generally GLBasic licence code is kept in 'AppData\Local\Temp\glbasic\glblicence.inc' file, that's generated with project compilation, it contains 'encrypted/hashed' email and licence code. Old (standalone) version code differs a bit from Steam version but after decoding should be usable to use in pre-Steam version.
In short: maybe ask Gernot for proper licence code for old version (submit your Steam profile for verification), decode licence.inc to use Steam credentials in older version (not sure how difficult it will be), or try using my code editor (works with GLB v14/15/16, but I didn't test compilation for all platforms). Another workaround but not tested, would be compile any project with Steam version to get 'glblicence.inc', use properties-advanced to set that file to admin created with 'read-only' flag for other users, so your user account would not be able to overwrite/delete it when you would use older GLB version.