Returns the number of frames remaining in the match.
Example, called from hit_player.gml:
// this character deals extra knockback in the last minute of a match:
if (get_game_timer() < 60 * 60) { // there are 60 frames in one second
hit_player_obj.old_hsp *= 1.2;
hit_player.obj.old_vsp *= 1.2;
}