Hi Julia,
If you’re looking for the media library, it should be Media > Library in your admin dashboard. If that’s not what you’re referring to, can you revert to the previous theme you had temporarily, and take a screenshot of the “Gallery Button”?
Reference to sharing screenshots: https://docs.generatepress.com/article/using-the-premium-support-forum/#uploading-screenshots
It would be good to take a backup of your site before doing this.
Or, if you could describe where we can find this button and what it does, that would help as well.