For me, changing the GTK+ input method was as easy as right-clicking in any GTK+ dialog box and picking "Input Methods" → "System (Simple)", rather than having

For me, changing the GTK+ input method was as easy as right-clicking in any GTK+ dialog box and picking "Input Methods" → "System (Simple)", rather than having to mess with /etc/environment. That setting seems to have stuck even through reboots. I also highly recommend you look through the standard XCompose file, since there's a lot of handy symbols—like <Compose><hyphen><hyphen><hyphen> for an em-dash, <Compose><hyphen><greater-than> for an arrow, or <Compose><C><C><C><P> for ☭.