When you're in gimp select the layer of your hitcircle.png you want to have transparent and change the opacity to a value different than 100%.
If it has only one layer you want to to be around 50-70% and if it has multiple layers it could be anything - in most cases you would just cut something away.
Notice that your hitcircle has to use .png, other extensions don't know transparency!
For transparent slidertracks I believe that they're transparent on default but I could be wrong.