Aftermarket soundcards by Creative, TurtleBeach, Yamaha and so on used to include a soundset of midi instruments. Does anyone know of a current soundcard that includes midi instruments.  I haven't seen any that advertise the feature.

MS Wavetable is part of all PC's. Sound Blaster's also had Sound Font banks but all are pretty terrible by today's standards. It's not something you'd use for music. I think they are like 8 bit. 
The TTS-1 is Calkwalks GM player. 

Well... Kinda.
 
Creative Labs cards include a powerful hardware mixer, and with that the ability to load Sound Fonts. They do include GS soundfonts, or at least did when I bought one (X-Fi).
 
That is pretty much it for ones that are soundcards with it built in to any extent. If you want a GM2 Rompler, Roland makes them. The Roland SD-50 and Roland Integra 7 are both hardware Romplers with a full GM2 (and GS of course) set. So are the older Roland StudioCanvas/SoundCanvas products.
 
As mentioned, TTS-1 is Cakewalk's GM player and it was made with the Roland sounds.

If you need GM sounds TTS-1 is about the best you can get IMHO.
 
I have an Integra-7 and buying one for it's GM2 set would be a bit OTT!

Drums Bass Piano Strings Sound Center  TTS-1    
 
Looks like Music Creator 6 comes with some excellent midi playback Soft synths. Hopefully you have explored them first. If you need more go to the KVR site and browse around, there are some excellent free synths. My Favorite is Mr Tramp. 
 
Nobody around here has bothered with Sound Card midi for a decade at least. 
It's handy for previewing downloaded midi files, gives you a general idea of the song. 
TTs-1 has more to it then the defaults, spend some time with the GUI mixer and try things like editing the effects.  I always tune my Bass Drum a knotch.