Maryland resident here and I've spent decades researching knife laws here down to the smallest detail. I've compiled all of my findings plain English here:
http://weaponlaws.wikidot.com/maryland-knife-laws
Gravity knives are not illegal in Maryland. It is not mentioned in any part of the state's regulations on weapon carry or purchase, and unlike other states, law enforcement is not permitted to arbitrarily interpret knife laws, but rather must comply with the statute itself and rulings of the high court.
I can see why this misconception persists. Many poor quality "knife law" websites have references to a Maryland case on gravity knives, but fail to provide context. Here's the real story. The case is
Savoy v State in 1964. where a pimp was caught by two plain-clothes officers while carrying a gravity knife. Contrary to what is often reported, this case didn't actually make a binding ruling on gravity knives, but instead ruled on the manner that jury instructions were provided. There have been subsequent changes to the law as well as binding case law since that time which effectively make the
Savoy ruling irrelevant. These include rulings that the all non-switchblade folding knives are penknives and so are excluded from the law (
Mackall, 1978), the burden lies with the state and not the citizen to prove a given knife is not an exempt penknife (
Anderson, 1991), as well as some changes in the phrasing of the weapon laws when they restructured the criminal code from "Article 27, Section 36" to "Criminal Code, Title 4, Subtitle 1."