@Sherry -
If it’s GPL, the source code is supposed to be free and openly available. I’m not sure if it’s GPL or not, but I’m pretty sure it’s supposed to be if it uses WordPress or PHP at all. PHP is open source as is WordPress.
The GPL only requires providing source to those to whom code is distributed. Since every party to whom BackupBuddy has been distributed does, in fact, have the source, then the requirements of the GPL are satisfied.

By the way: the GPL does not require anything to be provided free of charge. The GPL explicitly allows for charging a fee for GPL code, and further, allows for nominal charges to allay the cost of the medium on which source code is conveyed upon request.

But, if you're not a party to the distribution of the code in the first place, nothing in the GPL applies to you.