I think that running gpk-application from your "normal" user means it inherits that user's environment. That is different from running
from a terminal, as
Make the shell a login shell. This means the following. Unset all
environment variables except `TERM', `HOME', and `SHELL' (which
are set as described above), and `USER' and `LOGNAME' (which are
set, even for the super-user, as described above), and set `PATH'
to a compiled-in default value. Change to USER's home directory.
Prepend `-' to the shell's name, intended to make it read its
login startup file(s). Additionaly `DISPLAY' and `XAUTHORITY'
environment variables are preserved as well for PAM functionality.
(info coreutils 'su invocation'). In fact, running simply
(i.e. no dash) preserves the user environment, including the "proxy" environment variables