Note to King Lear, 1.4.209

King Lear,
Act 1, Scene 4, line 209
which if you should: i.e., and if that turns out to be the case. —Just before this Goneril said that she was "fearful" that King Lear was encouraging the disrespectful behavior of his knights; she now says that if he is encouraging such behavior, there will a punishment which may be humiliating to him. She doesn't quite make a direct threat to her father and king, but it's clear that she believes herself to be the responsible adult who has to deal with King Lear's irresponsible behavior.