In how far are CCR and GCR C*-algebras interesting?
The fact that GCR C$^*$-algebras contain the compact operators implies that they have minimal projections; this makes them behave in a way that somehow resembles matrices. Also, their spectrum is Hausdorff. These characteristcs made them easier to understand, and so in the 60s and 70s people studied them. So, it is not that much that they are interesting but rather they were the first ones that people could study.
The intersection with von Neumann algebra theory is negligible. The double commutant of the image of a GCR through any representation is type I, and these have been completely understood since the 50s.
Finally, you'll be hard pressed to find a current paper in main C$^*$-theory that mentions them. They do appear though in related works.