We have a GMSK envelope source in Spectrasys. This is accessed through the 'Multisource' part. This is a time independent source, however it will give users an approximation of channel occupancy.
If you are interested in a time dependent model our SystemVue product has modulated sources. Several types are built in and others can be created. Furthermore, this product is integrated with Spectrasys and will allow users to co-simulate the digital and RF. You can get measurements like EVM and BER.
Regards.