Is there a way to open a GUI when a player sits in a seat?

You could use a Changed event to detect when the occupant property of the seat changes to a new humanoid. When the property is nil, the seat is empty. When someone sits in the seat, the occupant property gets set to that player's humanoid.

Example

local seat = LOCATION_OF_SEAT

seat.Changed:connect(function(property)
    if property == "Occupant" then
        local humanoid = seat.Occupant
        local player = game.Players:GetPlayerFromCharacter(humanoid.Parent)
        if player then
            -- code
        end
    end
end)