In Bug 644244, comment 7 noted that two finger horizontal scrolling in the register did not work in GnuCash 2.4. As far as I can tell, on MacOS it still does not work. Horizontal dragging only causes the register to advance. To reproduce, shrink the application window so that the register is not fully displayed from side to side. Now, try to use two finger scrolling to move side to side. The register only moves up and down.
Just tried gtk's testscrolledwindow and horizontal scrolling works there so we've missed something in moving the register to Gtk3.
This is still an issue in GNUCash v 3.10 (Build ID: 3.10+(2020-04-11) running on macOS v 10.13.6.