This is not a settled matter, but I think that having a GPL-licensed library in your history is not going to be a big problem.
The GPL does not taint or infect other code, in the sense that it would force that other code to be GPL ever after. Instead, it merely requires that when you distribute a program that includes GPL components, that you also offer the corresponding source of the entire program under the GPL. Other components don't actually have to use the GPL license, but could use any compatible license.
If a program includes both GPL components and GPL-incompatible components (such as proprietary software) then the only legal consequence is that the program cannot be distributed at all. For example, purely internal use of such undistributable software is perfectly fine.
We can therefore say:
- the current version of your software is definitely not subject to the GPL
- at most, the versions that included the GPL component could be subject to the GPL
- in any case, it's probably a good idea to prevent internal Git history from becoming public, ever, even if you want to open-source a later version
Whether the past versions could be subject to the GPL, if they are ever distributed, depends on how the GPL component was used. A Git repository is not necessarily a single program but will generally be an aggregate of multiple works. In such an aggregate, GPL-covered programs could live alongside non-GPL programs without affecting each other. However, if the GPL component was a library, then your proprietary software might have been a work based on the GPL component during those versions, and would be subject to the GPL.
If you want to publish the source code of your proprietary tool, it's probably a good idea to either push through a one-time history rewrite to remove any doubt about possible GPL influence, or to start a new repository with a snapshot from your current state. Publishing internal history is usually a bad idea, independently from licensing concerns.