Tux Guitar DOES do Bass tabs... you have to define the staff as Bass Clef and the instrument as four string and set the tuning if different from standard.
Do the staff in the Clef item of the "Composition" menu and the instrument in the Properties item of the "TrackŁ menu
You can input notes using the fretboard view if you find that more convenient.
See screenshots:

setting the clef

setting the instrument in track properties

playing a Guitar Pro file
It also reads and saves GP3/4/5 files as well as imports/exports MIDI.
The current version has a bug in that when loading in a file, it reverts the staff to treble clef so you have to reset it.