When you receive a GPL'ed binary three primary rights are conveyed to you.
- You can redistribute the binary.
- You can request/obtain the source code.
- You can redistribute the source code.
Note: If you choose to redistribute a binary that has been compiled from modified source code you also have the obligation to distribute the modified source code - if requested by the person you sent the binary to.
If you modify and compile a program, you can choose not to distribute it at all - meaning you chose not to send any part of the modified source code or binary to anyone else. Assuming you do this - don't release anything - no one can compel you to do so under the terms of the GPL.
You could consider this to be "Private GPL", however it is the fact that you don't redistribute it that makes it "Private". As such you can combine it with any number of additional GPL projects - as long as nothing is redistributed - you can't be compelled to release any of it by the GPL.
You may also consider the situation of work for hire on unreleased GPL code to be "Private GPL" where someone is an employee of a company, they enhance an open source product - using the companies hardware, where they are being paid for the work and deploying only to internal servers of the company (no public access). Specifically it is private as the company has no intent to ever release it.