Wow, Jon, well done!
By gradient, we refer to uneven backgrounds.
These are typically found in light-polluted shooting environments or when the Moon is out.
What happens is that you end up with a light background tending to dark. Hence, a gradient.
You should head up to the pony club on new Moon weekends if you can get away from work. You'll really enjoy it and get to take some nice pictures.
All my early work was done there (actually at the paintball place at Kulnura before we migrated to the pony club at Mangrove Mountain).
Nicely done on your first attempt -- much better than my own attempts.
H