I think you're mixing up a couple of definitions.
Intelligent mode refers to an operating mode of the Roland MPU-401 MIDI interface, which is a standard for interfacing computers with MIDI sound modules. Many sound cards that claim to be MPU-401 compatible don't implement this mode, only "dumb" UART mode, which causes problems in some games that expect an MPU-401 compatible to be running in intelligent mode.
GM on the other hand has nothing to do with the interface between PC and MIDI device, but is a specification for MIDI sound modules in order to achieve cross-compatibility between them. It's basically a list of features and definitions that a synthesizer should have in order to be called GM-compatible. For example, instrument #0 should be an Acoustic Grand Piano, while #32 is an Acoustic Bass. Therefore, using a particular MIDI interface will not make a sound module GM-compatible.
The MT-32 can be made partially GM-compatible by sending a System Exclusive MIDI file that re-arranges the instruments on the MT-32 in the GM lay-out, but this is only part of the GM specification. Therefore, I'd really recommend you to use the MT-32 for games that specifically support it and use a GM-compatible sound module for later games with GM support. A Roland SC-55 or compatible would be the most obvious choice, although a Yamaha DB50XG or DB60XG daughterboard on a sound card is also a popular choice.