I'm working on a story called Moon Over Black Mesa. It's a Gamer Luna story where she plays Black Mesa: Source. I just posted the prologue last month, & am gonna post the next chapter when the Community Hazard Course Remake is finished.
For me, I think this is great because the MLP characters do not really fit their HL2 counterparts AT ALL. Just the opposite! XD Luna talks a lot, and loudly, whereas Freeman never does. Derpy is uncoordinated and clumsy, compared to Grigori who is a careful planner. Can't comment on Scratch since we haven't really seen her aside from fandom theories.
I don't perceive loud voice as so important to Luna character, rather, it's more like a legacy. In general, she is serious, stand-alone and introvert, which is quite what you can expect from Gordon. Also she has some class =] Grigori, on the other hand, is a little bit nuts, which plays funny with Derpy's personality. Only Derpy could have survived in Ravenholm after all!