You can import older projects (like .gtp) to Guitar Pro and save them in the latest version supported in your program. So for .gp5 files, you would need version 5 of Guitar Pro. Some 3rd party music notation programs may also be of help here, but using the original software is always best.
Convert Guitar Pro project to Guitar Pro 5 project.
We found one software available in the database for this conversion.
Microsoft Windows software - convert gtp to gp5 on Windows
Multi-track editor and music composer for guitars
Convert from gtp file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.