People say it's normal, but I tend to disagree. We have 38 GTPs right now, plus several on loan, and I've had over 60 in the past 6 years. I've also cared for many more aty pet shops, wholesalers, etc. I won't speak for every single GTP I've cared for since the '90s, but as far as my animals are concerned- every single time I have ever seen one of them on the cage floor, I could tell you why. Whether it was too hot, too cold, stress, illness, ate too much, running from a male, improper perches (or new ones it didn't like), etc... I always knew why.
Most people will say that it's nothing to worry about, and maybe it's not in this case, but I do not think it's normal behavior by any means. On most days I check on myu snakes several times in the day (and oftenh at night too, so I see a lot of their behavior. I have never one time seen one of mine just ground itself for no reason whatsoever.
You say temps and humidity are "normal". Well, what's normal? What is the thermal gradient (cool end to warm end), and what is the hot spot temp (if there is a hot spot)? Are those dowel rods for perches? If so, check them for mold and/or mildew. I recommend you use PVC perches, or if you insist on going natural, try bamboo or crap myrtle branches.
As far as the size, shape and placement, your perches look good, so I kinda doubt that's the problem. And again, there may be no problem. I'm just lending advice based on my own personal experience. Has anything changed recently? Any changes in the environment or other changes in his behavior? Anything that might be stressing him out? Had you just recently fed him a big meal? Is he in shed?
There seems to be a lot of reasons why GTPs will ground themselves, and most of them are no biog deal and/or easy fixes. The point is that in my experience, there IS always a reason, and you at least want to look into in order to prevent other potential problems. Somehting that is a minor short term problem could become or cause a more major problem in the long run. I hope this helps, and BTW, he is gorgeous! Happy Holidays! -p-
P.S. I really recommend using live plants. There's nothing really worng with most artificial ones, but live plants are very beneficial. They increase the humidity and air quality, they create cover and textures that lower stress, and they look awesome. If you want some good info on using live plants (what types, care, etc.), please feel free to send me a message here, and I'll gladly help you out.