How do I tell a script to find a model within a certain range of it?

well, it depends, if you want to know when a specific part is in range, than this should work;

local function in_Range(part1,part2,range)
    if (part1.Position - part2.Position).magnitude <= range then
        return true
        -- alternatively
        -- return part2
    end
    return false
    -- alternatively
    -- return nil
end

debounce = false

while wait(0.5) do

    if in_Range(game.Workspace.gate,game.Workspace.car,30) == true then
        if debounce == false then debounce = true
            game.Workspace.gate.CanCollide = false
            wait(3)
            game.Workspace.gate.CanCollide = true
        debounce = false end
    end

end

or, if you want to see if any part in a model is in range, then replace the last part with this;

while wait(0.5) do

    if debounce == false then debounce = true

        local carModel = game.Workspace.carModel:GetChildren()

        for i = 1, #carModel do
            if in_Range(game.Workspace.gate,carModel[i],30) == false then
                debounce = false
                break
            end
        end

        if debounce == true then
            game.Workspace.gate.CanCollide = false
            wait(3)
            game.Workspace.gate.CanCollide = true
        end
    debounce = false end

end

or if you want to know when any part at all is in range, I hate you.