I believe you need to buy it. That is, Twilight is not just a patch. It's an expansion pack. If you download the expansion pack, you will download version 2.01. That download can be made at no extra cost if you buy the CD.
Open Impulse. Click the Explore tab. Click Games. Click Strategy. You should see GalCivII - Twilight of the Arnor listed for $29.95. Click that and you'll be given three options for purchase: 1) Digital download of Twilight for $29.95. 2) Digital Download of GalCiv II - Ultimate Bundle Pack for $59.95. 3) Twilight digital-download + CD for $29.95.
Since I already had GalCiv II Gold Edition, I chose the third option. The Twilight CD was shipped to me (cost of shipping = $10.00), and Twilight v.2.01 became listed in Impulse/MyGames.
Important! --- Before downloading Twilight, make sure you have installed GalCiv II v.1.53 and Dark Avatar version 2.01. The first time I downloaded Twilight, right after buying it, I had older versions of GalCiv II and Dark Avatar installed, and I could not open Twilight. So I used WindowsXP/ControlPanel/AddRemove to uninstall Twilight. Then I went to Impulse/MyGames and downloaded GalCiv II v.1.53 and Dark Avatar v.2.01. Finally I used My Games to download Twilight v.2.01 a second time. It now runs perfectly.