When you first bought the software, the developer, Arobas Music , sent a confirmation email containing your license. Searching for "Guitar Pro" or "Arobas" in your mail is the fastest way to find it.
If you've lost access to your email and account, the Guitar Pro Support Team can often recover your key if you can provide proof of purchase or the original billing name. The "Free" Path: A Cautionary Tale guitar pro 76 license key full