How to set global environment variables in Guix System

[Lo Peter]

Dear All,

I am trying to setup ibus-rime for Chinese input in guix system 1.2.0,
after a lot of googling and trial and error, I found that I cannot set
the needed environment variables:

export GTK_IM_MODULE="ibus"
export QT_IM_MODULE="ibus"
export XMODIFIERS="@im=ibus"

I have tried putting them in ~/.profile, ~/.pam_environment, ~/.xprofile
and it seems they have no effects.

If I am not mistaken, guix system now uses gdm as display manager, but
I also cannot find a way to set environment variable for gdm sessions.

Any help is greatly appreciated. Also, I think a way to let user
customize /etc/environment through the system config will be useful,
but I do not find a way to do so.

Thanks.

Regards,
Peter