GP6 - May I install Guitar Pro 6 on more than one computer?

YES. The purchased license is acquired by the end user with no time limit, and gives him the right to install the software on a maximum of 5 CPUs or operating systems over a 12-month period(*). 
It is understood that the use of the software, even when installed on 5 CPUs or operating systems, is for a single user and, by extension, to his/her family (same household). This makes it possible, for example, to reinstall Guitar Pro in case of (a) new installation(s) / change(s) of operating systems, or in case of a double installation on a desktop CPU at home and a laptop belonging to the same user.

(*) You have 5 activations available. For each one used, the total count of activations drops by one. One year after each activation, the total count of available activations is augmented by one.