1 gear is the term for someone who doesn't catch onto jokes quickly, or has no common sense.
This person that inhibits '1 gear' doesn't necessarily have a learning disability, they are just slow on catching on. They are completely aware of their intelligence but it takes them a couple seconds or a couple minutes to realize what is going on in a conversation.
Usually occurs when someone is trying to tell a joke and they don't get it. Or, when the person continues a conversation and the opposite person has no idea what their talking about after the conversation has continued.
Larissa: You cried too.
Nina: I'm not fat.
Larissa: ....it hurt you too.
Nina: I'm not fat, I'm just big bone.
Larissa: What are you talking about? Huh?
Nina: Oh...I thought you were talking about me calling her fat.
Larissa: 1 gear Nina.
Nina: What's the number for 911??
Nina: No! The number for 911!
Larissa: NINE ONE ONE
Larissa: 1 gear.