Can I be held responsible if I let the dealership take back the vehicle that my late husband was leasing?

First, please accept our sympathy for your loss.
MD is not a "community property" (which also should be called a "community debt") state, which means that you are not personally liable for the vehicle even if you made some payments for it, so long as you did not take over the lease (assume it, or have it assigned to you; i.e. have it put in your name) and never co-signed or guaranteed the original lease.
GM can try to recover from your husband's "estate" or the money and assets he left behind--those assets, etc. are supposed to be used to pay your husband's debts and obligations before being distributed to those (e.g. you) who will inherit. Some assets cannot be reached, because they don't become part of the estate: real estate owned by him and you as joint tenants with right of survivorship; joint accounts, or accounts which were pay on death (POD) or transfer on death (TOD) to you; life insurance naming you specifically as the beneficiary; etc. Those things go directly to you without going through his estate. But any thing he owned separately from you, like if he had bank accounts solely in his name which were not POD or TOD to you, for example, is something GM could seek payment from. But the most they can get is what is in his estate--if there was little or nothing passing by way of the estate, the will not be able to recover anything.