Make changes to be applied after guix install and system reconfigure?

[znavko--- via <help-guix@gnu.org>]

Hi, Guix! I go to try programming stuff. And I have a question to ask.
When I install some essential system packages or also reconfigure system
the changes do not go to be applied on the fly.
F. e. trying to work with dates in C I have such a 'history':

 444 date -d 1306830000
 445 glibc
 446 nano t.c
 447 gcc t.c
 448 guix install gcc
 449 guix install build-essential
 450 nano t.c
 451 gcc t.c
 452 nano t.h
 453 nano t.c
 454 cd /home/bob/Videos
 455 ffmpeg -i 1.mp4 -ss 00:00:02.90 -to 00:00:40 -c copy 2.mp4
...
 504 cat t.c
 505 gcc t.c
 506 ls
 507 ./a.out
 508 history

So here I install "gcc" and "build-essentials" but have no success cause
my simple copypasted lines do not work without reboot.
And only next time I turn back to this interest after several restarts
I have success.

Please, can you advise me what to do after installing such system things
and also after system reconfigure for get changes work?

PS: my code was without errors but did not compiled because off linking,
due to I've just installed gcc and did not rebooted.

$ cat t.c
#include <stdio.h>
#include <time.h>

int main() {
 char text[100];
 time_t now = time(NULL);
 struct tm *t = localtime(&now);
 strftime(text, sizeof(text)-1, "%d %m %Y %H:%M", t);
 printf("Current Date: %s", text);
 return 0;
}