In order to enter G-mode, Samus needs to take damage after touching a room transition so that she immediately takes damage and triggers the refill while entering the next room.
In addition, she needs to be using X-Ray while entering the room and release X-Ray before reserves have finished filling.
For the simplest cases, Samus will need to start with 4 (or fewer) Reserve Energy, giving a 4-frame window to release X-Ray in the next room (or fewer: 1-frame per Reserve Energy).
When successfully entering G-mode, the room will darken and the door will remain open (releasing X-Ray too early, neither of these occur; releasing it too late, the room darkens and the door closes - this is R-mode).
See also: Tech Page