PyInstaller is distributed under the GPL License but with an exception that allows you to use it to build commercial products:
You may use PyInstaller to bundle commercial applications out of your source code.
The executable bundles generated by PyInstaller from your source code can be shipped with whatever license you want.
You may modify PyInstaller for your own needs but changes to the PyInstaller source code fall under the terms of the GPL license. That is, if you distribute your modifications you must distribute them under GPL terms.