According to user Retroblazer, Doom 64 for PC comes with "a whole new episode". In this thread, we will investigate and ultimately come to the conclusion on how this new episode is accessed.

Upon starting a New Game, I am only given the option to select a difficulty level.

Do you need to beat Doom 64 first before this new episode is unlocked?

Aside: I tried searching these forums for the phrase "Doom 64" and received no results, even though I can clearly see the phrase verbatim in some thread titles.

Upon careful application of a chosen search engine, there appears quite a rabbit-hole on this topic.

Plus, you've succeeded in selling me Doom64 on GoG, so there's that :|

This hunt looks like being fun, so no spoilers from me.

Yes, you need to beat base Doom64 first.

Wow, Doom64 is awesome, and tremendous fun, but a little too hard for me.

On the N64 original, I never came near the secrets, let alone finishing.

On the plus side, shall _definitely_ return to this thing and its secrets...
once I've earned my gaming stripes on Doom 1, then 2.

I don’t think you need to beat Doom64 first if you use the Doom64 EX+ port. I’ve been so impressed with CrispyDoom that I’ve downloaded all my Gog Dooms and tried source ports for them. I got around to Doom64 today and noticed in Doom64 EX+ that on starting a new game it offers the standard and lost episodes (when using the WAD from Gog Doom64).

Bit late but I hope this i go helps someone.

Thank you for the info, much appreciated, it's really good news.