By using a custom sign-up form you can provide a standalone Google login. The button would need to be custom coded to point to the right URL.
Here is the link to Wix article on custom forms:
https://support.wix.com/en/article/creating-a-custom-signup-form-for-your-members-area
There is a bit more information in this thread, including a code example.
https://www.wix.com/corvid/forum/community-discussion/login-using-facebook-button