You don't have to Import .dat over .gp4 file, just to copy the .dat in the same folder as the .gp4 and open the .gp4 in 3DEditor then both will be open.
Note about both .gp4 and .dat display 3DEditor can now read and edit track physical side along track graphical side. To avoid any trouble and file corruption (if ever), this feature needs to copy the .dat file associated to the .gp4 track in the same folder. If the .dat file associated is found, it will be open with the .gp4, else, only the .gp4 file will be open.