Just posting in case it helps anyone here -- I just installed and was getting the g1.dat error repeatedly, despite selecting the proper folder (the one containing Data/ and ObjData/ and thus g1.dat) at install. I had a fresh download of RCT2 from Gog with verified integrity.
What I eventually found fixed it was going to OpenRCT2/Documents/config and changing game_path to the correct location -- it was set to my existing RCT1 installation in a different folder, despite me never selecting it! At some point, the install was overriding my manual folder selection and re-defaulting to the rct1 location. rct1_path was left blank.
Hope this helps someone; let me know if it would be helpful to file a full bug report.
