Unit Takes Damage

Is there a way with GUI to make a trigger that if a unit owned by Player 1 takes damage it is triggered?

Yes it have, but it is laggy and need 3 triggers:
Trigger 1
Events - Map initialization
Actions
Unit Group - Pick every unit in (Units owned by Player 1 Red) and do (Actions)
Loop - Actions
Trigger - Add to Trigger3 <gen> the event (Unit - (Picked unit) Takes damage)

Trigger 2
Events
Unit - A unit enters (Playable map area)
Condition - Owner of Entering unit is Player 1 Red
Trigger - Add to trigger 3 <gen> the event (Unit - (Entering unit) Takes damage)

Trigger 3 is the trigger that you insert the actions:
Events - None - they are added by the other 2 triggers
Actions - Your ones, use "Triggering unit" to reffer to the damaged unit

Great! This will help me plenty, thanks!