He got him in the choke because Gohan wasn't fully in the fight, like Piccolo brought up at the start. Gohan successfully landed the only strike in their bout, whereas Piccolo's rear knee attempt was blocked even though Gohan didn't know where Piccolo would be attacking from. Gohan dodged Piccolo's green ki wave slasher nearly point-blank, which is when Piccolo lambasted him for not taking the fight seriously enough. That's when he got Gohan with his arm, choked him out, and mentioned how he lets his guard down at vital moments.
I don't see anything in the fight that would make me think Piccolo was actually stronger than Gohan ki-wise, let alone noticeably stronger as some claim. Piccolo capitalized on Gohan's weaknesses, that's all.