Gravel rash is a term used in the gay community, to describe their first anal sex experience, as often the sphincter is very tight, and may produce a rash on the tip and shaft of the erect penis.
Trent: "Man, my first anal sex with a guy gave me the biggest gravel rash on my fucken knob aye! Luckily with time, the wounds have healed and so has my gay sex life!"
The horribly painful rash one gets after tripping and falling, or 'pavement surfing' as it's often called.
"I fell on my face and now I have gravel rash on my face, down my arm, down my leg and on my arse"