invActive

В step:

if keyboard_check_pressed(vk_tab) {
    invActive = !invActive;
    event_user(3);
}

В User event 3:

/// @description Обновление инвентаря

for (var i = 0; i < array_length(inv); ++i) {
    if (invActive) instance_activate_object(invCells[i]);
    else instance_deactivate_object(invCells[i]);
    invCells[i].obj.id = inv[i].id;
    invCells[i].obj.sprite = inv[i].sprite;
    invCells[i].obj.name = inv[i].name;
    invCells[i].obj.stack = inv[i].stack;
}