Wpa supplicant

Can be enabled on NixOS with networking.wireless.enable = true.

Extra configuration can be specified inside networking.wireless.extraConfig.

Wpa_supplicant_gui

To be able to use wpa_gui as user put the following in your configuration.nix file:

networking.wireless.extraConfig = ''
  ctrl_interface=/run/wpa_supplicant
  ctrl_interface_group=wheel
'';

Also your user must be part of the wheel group (replace USER with your username):

users.extraUsers.USER.extraGroups = [ "wheel" ];

From the shell terminal, use the wpa_cli command line tool and specify the network interface device with -g

wpa_cli -g /run/wpa_supplicant/wlp3s0
list_network
select_network 2

As a means to debug if things are working, open another terminal and examine the logs by:

journalctl -u wpa_supplicant -f