I think you are mistaken that git itself asks you for your credentials in a separate GUI window. git
is a command line only program. However, there is the credentials subsystem which in turn may call/calls some external program to do its job.
You need to check your git configuration file to find out which program is invoked and change that setting. See the git doku on Credentials Storage for reference. Don't be fooled that this help is about the storing of passwords. The thing you need to look for is disabling the credentials helper (restoring the default behavior with no credentials helper) to get what you want.
Here is how to remove the helper:
git config --<system|global|local> --unset credential.helper
More about how to uninstall the credentials helper for windows.