GP6 - Can Guitar Pro 6 open files produced with the previous versions of Guitar Pro?

YES, you can open gp3, gp4, and gp5 files. (The gtp format (Versions 1 and 2) will also soon be available in Guitar Pro 6.)

All files will be saved in the .gpx format, and you can export to the .gp5 format as well (menu File > Export > GP5...).