Hi there,
Try this:
1. Create a new Block Element:
https://docs.generatepress.com/article/block-element-overview/
2. Add a GB Container Block, with an Advanced > Additional CSS Class(es) of: hide-on-desktop hide-on-tablet sticky-mobile-footer
3. Add your styles and button to the Container Block.
4. Set the Block Element Hook to after_footer
5. With the block published, then add this CSS to your site:
.sticky-mobile-footer {
position: -webkit-sticky;
position: sticky;
bottom: 0;
z-index: 100000;
}