Hey Maxi,
It looked like something fun to figure out, so I created this feature with the slider, as you suggested.
Take a look at what it looks like:
Download for the example:
IOWA Gambling Task Progress Bar.zip (132.2 KB)
Explanation:
At the start of the experiment, we set 2 variables, one for the wins and one for the loses:
# Begin Experiment:
win_sum = 0
lose_sum = 0
We have two buttons in the experiment, one demonstrating wins (+100) and one demonstrating losses (-100). Whenever one of those buttons is pressed, we increase the corresponding variable by 100:
Win Button: win_sum += 100
Lose Button: lose_sum += 100
When either of the buttons is being pressed, a new routine starts.
Whenever a new routine starts, we set the win_lose text to:
$str(win_sum) + " Won | " + str(lose_sum) + " Lost"
Meaning we will get the updated variables in our text component.
Now for the sliders.
We have a green slider (wins) and a red slider (loses) with the following settings:
And readOnly ticked so the participant won’t be able to modify the slider:
At the start of each routine, we set the starting value of each slider to the corresponding sum:
green.startValue= win_sum
red.startValue = lose_sum
It should do the work, and in my opinion, it looks great 
I hope it helps and gives you a solid direction!
Chen Brilling