Is it possibble to transfer the save file from the steam version and use it on the gog version?

In theory, the saved game files should be cross platform from steam and gog, but doing a cursory look for them on my hard drives, I cannot locate where the gog saves are found, so I cannot tell you how to transfer them.

C:\Users\ * your user account name * \AppData\LocalLow\InXile Entertainment\The Bard's Tale Trilogy\saves

Gog save games should be in the above folder.... .

For Bard's Tale IV the location is actually:

C:\Users\<user name>\AppData\Local\BardsTale4\Saved\SaveGames

If the location is in AppData at the path shown above, you wouldn't even need to move the saves. It should be a straight 1:1 transfer. I'm not paying $35 to test it though XD