Some people (including myself) sometimes use the free CoolSoft VirtualMIDISynth and our favorite soundfont(s) as a substitute for the Microsoft GS Wavetable Synth. VirtualMIDISynth doesn't have any sounds of its own; it needs one or more soundfonts. I think a lot of Yamaha users who use VirtualMIDISynth like to use Don Allen's Timbres of Heaven Soundfont with it:
http://coolsoft.altervista.org/en/virtualmidisynthhttp://midkar.com/soundfonts/index.htmlBut there are other MIDI players and GM1/GM2/GS/XG soundfonts available, so you don't have to use VirtualMIDISynth and Timbres of Heaven if you prefer something else.
By the way, you should be aware that soundfonts which bill themselves as GM1/GM2/GS/XG compatible do not necessarily-- and probably do not-- contain the same soundwave files as a Roland GS or Yamaha XG keyboard, because those soundwaves are proprietary and copyrighted. Rather, it is similar to the way that the GM1 soundset works-- GM1 specifies what instrument each Program Number should sound like (Acoustic Grand Piano, Nylon String Guitar, Upright Bass, Alto Saxophone, etc.), but each company that manufactures a GM1-compatible keyboard uses their own soundwave files for these GM1 voices, such that for instance the GM1 Acoustic Grand Piano voice doesn't sound exactly the same on keyboards made by Casio, Korg, Roland, Yamaha, etc. All it really means when someone says that a soundfont is GM1/GM2/GS/XG compatible is that you can send the Bank Select and Program Change numbers used by those soundsets to the soundfont and it will play a voice that is intended to correspond to the requested GM1/GM2/GS/XG voice.
As for the Microsoft GS Wavetable Synth, it usually does show up in a DAW under the list of available MIDI Out devices (ports). But I'm not familiar with LMMS; I do have an old version of it installed on my Windows 10 desktop computer, but I was never able to get the hang of using it-- although I admit I didn't put much time and effort into trying since I already had other DAWs that I was able to get around in.