Suggestion: the Search key under your left pinkie emits SuperL (aka. Meta, same as a Windows key), and it is an great way to make up for some other keyboard weirdness Chromebooks have, and map to WM controls.

I recently discovered keyd, an excellent system-wide key remapper that works as a tiny daemon that intercepts input events and re-emits them as a virtual keyboard, and have it mapping Search+Arrows to PgUp/PgDn/Home/End (like a lot of laptops do with Fn+Arrows, or ChromeOS does with Ctrl+Shift+Arrows). I've already run into a couple other folks doing the same because it's such a clean solution to the Chromebook keyboard.

AFIK GalliumOS has been unmaintained for over a year, and most of the patches they used to add are now in mainline, so long term you may want to consider a different distro - it's probably OK for a while still though.