They both feature a really eccentric gyaru who, not only does she solve the occasional mystery, but they also have close ties to the police force. Kechonpa is more mature, and heavy-handled (especially with it's dialogue), while GALS! is a light shoujo.
The art style is similar. You know the old timish style where all the characters look the same except for the hair and feminine/masculine features. Also, I'd say Arisa (the alter ego) and Ran are alike except Arisa picks on people for no reason at all.