Hope the gravel holds up.
Stepdad built a brick grill/smoker with lots of extras. We set 6 4X4X10 TP post without gravel, concrete or extra treatment. Poured a 10X12 slab and built a simple roof over it, except for the chimney. Added a 1X12 shelf on both sides waist high, 30" from the slab. 10 years later the shelf is at 24", 15 years it's at 19". When he died I couldn't stand under it. Mom sold the place and I never went back.
Run a 2X10 or 2X12 on the slab to post, 3 bolts per post. It will serve as a bumper and a termite barrier. Might keep a post from sinking.