Apple - How do I disable Safari's Show All Tabs gesture?

You can easily disable this by installing BetterTouchTool, and then for Safari adding a new Touchpad gesture for Pinch Out with no action.

Here's a screenshot:

This is currently not possible. Apple does not provide a way to turn off this gesture.

Note that if you pinch out whilst still zoomed in slightly, the tab switcher is not invoked.

Yes, you can disable it by going to System Settings → Trackpad → Scroll & Zoom and then uncheck the Zoom in or out box. Of course, by doing this, you also won't be able to pinch to zoom in and out on webpages and in other apps.