When I have a Cutscene Camera / Camera Interpolation, can I add a gui in it?

You could toggle the visibility of the gui when the cutscene is triggered.

--Local Script

local plr = game.Players.LocalPlayer
local plr_gui = plr.PlayerGui --All players have their own folder that holds anything you had in StarterGui, so say you put a gui in StarterGui, it'll appear here when they join.
local ui_element = plr_gui:WaitForChild("put the name of the gui here")

function on_cutscene()
    --cutscene stuff here
    ui_element.Enabled = true --If it's a ScreenGui then use this
    --ui_element["Certain ui element here"].Visible = true --If it's NOT a ScreenGui then use this
end