You can either buy it or download it illegally using torrents.
Main Gear:

ESP Eclipse CTM I
Dean 'Shards' Razorback
Chapman Guitars ML-1
Ibanez AEL40SE Acoustic
Blackstar HT-5 Mini Stack
EHX Metal Muff
EHX Small Clone

In short, it's a tabulature editor, that allows you write a tab, and then make the computer play what you just wrote. You can have multiple different instruments playing at the same time. Some people use it to write and arrange original songs, while others use it as a sort of advanced tabulature (Like normal tabs, only that instead of just writing a tab, the author of the Guitar Pro tab recreated the entire tabbed song in Guitar Pro).

You can download a trial version at the official website ( http://www.guitar-pro.com/en/index.php ).
Tuxguitar. Google it. It's like guitar pro, but free.

Guitar Pro > Tuxguitar > Powertab.

Only tuxguitar and powertab are free.