Updated: Dec 8, 2019
Ever been to a MD dealer, they sell you a handgun and after you pay all the money, fill out all your paperwork, pass all the background checks, and complete the 7 day- you are all ready to leave the gun shop- they stop and tell you "Wait, you have to buy this gun lock before you leave".
No. No you are not legally required to do that. It is required that "a dealer may not sell, offer for sale, rent, or transfer in the State a handgun manufactured on or before December 31, 2002, unless the handgun is sold, offered for sale, rented, or transferred with an external safety lock."
You are legally allowed to bring an external safety lock you may already own and install it before the handgun is transferred to you. There are dealers out there that will prey on your ignorance and hide behind a false legal requirement.
Another fun fact, it's not unheard of for your firearm to arrive at a gun shop with an external gun lock, only to have it taken out and sold to you a second time- unbeknownst to you.
If they tell you this- ask if that is Maryland law or their store policy to blindside you to require you to spend more of your hard-earned money. You know the true answer to that already. You are not required by Maryland law to use a lock recommended by the Maryland State Police.
Know the law. This can be independently verified on your own. This is a part of Maryland Public Safety Code Subsection 5-132 (2015) (c) (1)