Not being a coder, I have no idea how to implement something like this into Renoise. Would it be possible to make something out of this code?
https://github.com/antonhornquist/gong
One can dream, no?
From what you have linked there the short answer is no. The slightly longer (more complicated) answer is that (to me) it looks like a WIP for a synth that uses some framework based on things like GitHub - monome/dust: norns incantations and SuperCollider (https://supercollider.github.io/) (?) In other words I imagine there is more external code involved than what you have linked.
Probably - I wish I knew more about it. Just dreamin’. I’m doing everything I can to not use plugins for synthesis, I like the low weight of the Renoise system 
Morphsynth is probably your best bet for basic FM synthesis in a Renoise tool.
I am also aware of Overtune which can do complex FM tones and looks quite powerful.
If you ever change your mind about using VST’s then there is always Dexed VST 
Thank you for the suggestions! I have MorphSynth and the rest of the FM plugins I have considered and/or tried at some point in time over the past 25 years. Stochaton is pretty nice and simple, and there’s JUCEOPL, an OPL-3 emulator. Beautiful sounds, not very stable in how it accepts notes/sequencing.
Having used Reason for the majority of my time, they popped out this one synth called “Europa”, and there was one function it had that really rocked my socks, and it was the simplified FM modulators it has. I have no desire to use Reason any longer, even if it has put out its synths as plugins. Just wishing and hoping we can have real, calculable FM in Renoise.
I have been working with the amplitude modulation in the sampler modulation section, and it is soooooooo close to what I’d like for it to do, just not fast enough, in mod-speed. Have also considered the folding distortion, and it’s not subtle. Ring mod is, well, ring mod. Not the same. Have tried the DC-offset synth, pretty nice, but really more of an analog synth method.
Just wishing for a Renoise-native FM synthesis method that can be hooked to modulation sources for timbral movement. I use the sampler as my primary sound source, so even something that could modulate the samples in a basic FM way would be a treat!