Hello Koen,
I may be too late with this answer to help, but just to confirm that "Goal" is not currently exposed to the Blueprints, as per CA PPM 15.4.
There can be various reasons why Attributes are not exposed, but this one was reviewed ( DE37624) and decided at the time not to be exposed, so there is no workaround that I know of for now.
It may be in the future, but you could also log an Idea here at PPM to request that this - or other missing ones - are added.
Thanks, Kyle_R.