While a GNS or GTN series product (with the proper software level) is what is most often used as a qualified position source, there are versions of the GDL-88 that include an internal WAAS receiver. Having a WAAS receiver is a requirement for ADS-B compliance. This will allow your exact position, speed, etc., to be broadcasted to other aircraft. Four versions of the GDL-88 are available:

GDL-88 Standard
GDL-88 with Diversity
GDL-88 with internal WAAS receiver
GDL-88 with Diversity and internal WAAS receiver