Deluxe Music can assign some midi channels to samples, that's right. But that doesn't mean it can play General MIDI files straight away. It doesn't even know what GM is, it was released before the GM Standard was defined.
You would need to manually pick up samples that suit the instruments and remove as many musical notes that it fits the 4-voice Paula "synthesizer". Pretty much the same as if you would use a tracker.
MIDI Playback can be damn expensive, much more expensive than an mp3, it depends what features are supported and what not. If you cut it down to MOD specs, like 8bit samples, 4 Channels and no effects what so ever, it can be played. But that requires manual adjustment to give reasonable output. If you do this automatically, the result will be probably very bad, depeding on the .mid file of course.
If you have a .mid file that uses only 4 voices at a time, it can be perfectly mapped to a MOD. If it uses 50 or 100 voices, you need to manually decide which instruments are less important an can be dropped, and where to "steal" the voice when there are not enough available.
With the 16 "channels" in my above posting I ment, that a device needs to be able to play 16 voices at the same time to be allowed to carry the GM logo.
Don't mix this up with the 16 MIDI channels, this is the "number of tracks" you can use, but they don't necessarily need to play all at the same time. On the other hand, unlike a MOD, one track can play as many musical notes as it wants, while a MOD can play only 1.
Btw, the MIDI format is much more powerful than MOD, this is a common misunderstanding. It just doesn't come with samples. If you use bad samples, it sounds ridicoulus of course, while within a MOD the samples are choosen by the composer and do always fit. If you don't use General MIDI, it isn't even defined what samples/instruments to use. MIDI was just never ment to share the instruments (and therefore be dependent on a certain format of handling instruments), just the musical notes.
E.g. when you compose music using real (analog) synthesizers, there is no sample involved during the generation of the actual audio, so how could you store this in the .mid file? Think of a .mid file like some musical notes printed out on paper.
