I will assume that you have developed this code de novo, and thus are the sole rightsholder in it.
Licences do not inhere in software, they attach to the recipients through the act of conveyance. So if you upload it to a repo that nobody can see or access, saying that you are doing so "with a GPL licence" is fairly meaningless. The important bit of this plan isn't the licence, it's the third-party date stamp, which could be used later to prove you'd written the code before you started working for the company in question.
Unfortunately, depending on your jurisdiction, the nature of your relationship to this company, and the details of your contract with them, this may not help. In most jurisdictions there is the concept of work-for-hire. This may or may not have the effect of giving the rights in your pre-existing code to the company; but if you work on it as part of your involvement with the company the codebase may end up hopelessly rights-encumbered, and be effectively unusable by you afterwards. In some employment contracts things are even worse, and the company becomes the rightsholder in anything you do while an employee, creating the above problem, in spades.
In short, the problem as I see it is not proving that you were sole rightsholder in this code before you started working for the company, it's proving that you're still the sole rightsholder after you've worked on it for them. We do have some questions here that address this, but it comes down to being contractually explicit about what rights in the codebase you are transferring to the company, and what rights you reserve. If you duck this question now, don't assume that you can pull a big reveal later, expose the earlier repo, and the company will just shrug and walk away. And, of course, IANAL/IANYL.