Gul-Dass is a very, very tricky champion to use. If not played correctly it can be truly awful.
Essentially it is a champion that needs to be protected, but at the same time it doesn't do much unless it gets in the fray and damaged. If you don’t give the opponent the chance to OTK Gul-Dass then any wound means it will hit a lot harder. At the same time there is really no need for the opponent to attack it as 2 melee attack strength is not very threatening.
So what is its real role?
Mainly it is a wall to protect your frontline and put pressure on your opponent. A wall that can suddenly do a significant amount of damage if Blood Summon or your own attacks are used against it.
However don’t make the mistake of exposing this wall too much, Gul-Dass is not life efficient (it just has a 1 to 1 life to magic ratio, which is quite terrible). If your opponent one-shots it, as they certainly will try to, your champion was effectively a negative value for you.
So the key is to protect all but one of its flanks, leaving just one exposed and letting it grow into a serious threat as it becomes damaged.