You can transpose anything in Guitar Pro 6. There are a few ways to do so but the easiest method is in the following linked video. Check it out and try it out yourself. Remember you will have to transpose each track doing it this way.
You can also go to Tools>Transpose and change the setting to "All Tracks" and "All Bars" and then select the amount of semitones you want to transpose it.
*This will only work with tab files in GP6 and not real audio recordings.