But taking up what you were talking about, the 'learning curve':
Perhaps not for everyone but generally, people need to have paid attention in middle school and high school in English class to grasp grammar, sentence structure, vocabulary, and have had practice in creative thinking. All this takes years to develop generally.
Depends on what you want, really. If you want basic one-lining, people need to speak a little English (although the translator will do in a pinch), need to be at least slightly below average in intelligence, need to have been taught and understood the most basic of the basics.
Anyone that isn't very stupid or doesn't speak English at all can learn this. Most 'gamers' could, if they wanted.
If you want half-decent semi-para, they need a lot more. The translator won't do, really. They need to be able to speak English and be able to at least read and understand complex sentences. They need to understand the basics of RP fairly well, probably need to be above average in intelligence, and some creativity is also desirable.
An intelligent person with a decent command of the language and a fair general education can learn this fairly quickly, the RP-stuff and the essentials of the lore being the hardest (IC, OOC, no metagaming, etc. and for Gor, the lore relating to the theme.) All fairly smart people with an open mind can become good semi-para RP-ers. That includes all smart 'gamers' that speak English.
The same minimum requirements apply for full para-RP, except that people need to be able to write long posts. And if those are going to have to be any good, and not just meaningless babble, or memorized para-fluffing, that rules out most non-native speakers. It rules out many without a flair for writing. It rules out a lot of 'silent types', and people without an ever-flowing well of at least semi-creative writing inside their heads.
Some people have that talent, and have learned to use it long before. Many para-RP-ers report having some 'amateur-writing experience' or background. I suppose it can be learned, but I am not sure if people can really learn to do this stuff well in just a couple of years, after the formative teenage/adolescent years. For many that come to this sort of writing late, it will always be a major effort, though some may have had no experience, but discover that they just had the talent already. It can't really be learned within a reasonable time-frame within SL, unless someone already had the skill (and required patience for it) already. Para-RP will always be a niche thing, because most people don't have the right screw loose for it. Probably less than 5% of the population of an English-speaking country would have the right combination of skills and personality to be able to para-RP and enjoy it, I'd guestimate.
So, effectively, the skill cannot be learned (or maybe only by a few). But some 'gamers' just happen to also be the sort that tried to write books when they were teenagers, or short stories, etc. Those play games and stuff too. So, if someone already has that skill, is 'wordy' enough, etc, they can still 'evolve' from a 'gamer' to a para-RPing player.