El Capitan app/extn to allow whole-screen zoom with just trackpad I need to do self deep tissue massage on my back each night.  I roll around on the floor on top of a tennis ball for a half hour.  During this time I watch stuff on my MacBook Pro, which is running El Capitan.  I control it with my Magic Trackpad.

I'm sufficiently far away that reading Finder items is a little difficult, so I'd love to be able to zoom the screen with the trackpad.  But the Trackpad System Preference is useless for this; it seems it only works with e.g. iPhotos.  Accessibility System Preference insists that a key be held down while doing the zoom gesture in order to zoom the screen, so it's useless too.

So, does anyone know of an application or extension for El Capitan which will do whole-screen zooms from JUST a trackpad gesture?  Googling turns up nothing useful :(
I use BetterTouchTool (BTT) for customizing a trackpad (or I did before my trackpad fell apart has yet to be replaced). I know that BTT can do what you are asking. Since it can also be used to customize most other input devices (keyboard, mouse, remote, etc.) and not just a trackpad I still make use of it without a trackpad and with or without I consider it one of my essential pieces of software. I've moved most of the my shortcuts to BTT so that all my alterations are in one place.

BTT allows you to set up most any gesture you'd like, not just the standard Apple gestures. The downside is that the documentation is somewhat sparse and it can help to have a certain amount of technical experience to figure out how to set up certain functions but your desired need shouldn't be too difficult to configure. 

I believe that you can download a trial version before deciding to pay for the app and the license cost is very reasonable and done as  a "pay what you can" sale (with a low minimum cost). (BTW, I'm only a user  of BTT and am not connected with it's development or sales.)

BetterTouchTool is available from BetterTouchTool.com which actually redirects to the developer's site: https://www.boastr.net/