Possible, but not as likely as the weapons being somewhere in the terminal.

The easiest way to get a weapon planted inside of the terminal is to find a low-paid worker with access and trick them into doing it for you. You'd want a low-paid worker as they'd be more likely to want to earn a few bucks and would be less likely to report it. The airline employees with access to the interior of the airplane have pensions, benefits, good wages, etc. that they'd be less inclided to risk for a few hundred bucks.

The tactic would be to find a low-paid contract employee and offer him a couple hundred bucks for planting a box of "pot" inside of the terminal. He'd be told that an acomplice would pick it up and carry it to wherever it's being smuggled. A person who wouldn't knowingly help a hijacker might be willing to help a pot smuggler if it meant a some bucks in his own pocket.

There are some contract employees with access to the aircraft cabins but the timing is more difficult, there are fewer places to hide the box and they don't really have the run of the airplanes as a janitor does of the terminal.

The chief defense to this type of attack is educating the ID holders, thorough background checks, frequent monitoring of likely hiding places in the terminal and aircraft and monitoring the ID swipes of employees looking for accesses that don't match the employees work pattern. The TSA has increased efforts in all areas.

The gate checks were put in place as a short-term measure while more permanant precautions could be implimented.