The indicator icon for the gamemode module takes up space even if use-icon: false is set. This can be noticed by launching GTK_DEBUG=interactive waybar, finding the gamemode image in the object hierarchy, clicking on it, and noticing how there is a small blue box next to the text. Alternatively, putting a background on #gamemode in the CSS shows that there is more spacing to the left of the text than to the right, as that extra space is accounting for the icon (see image). Setting icon-size to 0 helps but does not fix the problem entirely (setting icon-spacing to 0 seems to have no effect).