Okay, it's kind of complicated for Ultimate, but definitely doable.

So, unpack the whole file somewhere. There's a bunch of folders we'll get to in a second, three readmes and a userStartup.cheat.

Go to C/Users/Name/Documents/EA Games/The Sims 2 Ultimate Collection/Config.

There's the regular userStartup.cheat file. If you haven't changed anything about it, it'll only contain the order to turn off testingCheats on start. Either copy the text from Gunmod's file and paste it into your file or just replace the files. Remember to back up your file first. You can edit the .cheat files in the text editor. What the new file does is force your graphics setting to max and activate some stuff like Shaders. I'm not sure if you can't just change any of those in the Graphics tab, but apparently you need it. (According to the readme, you can turn Shaders off and not lose too much.)

Now for the folders. Go to C/Program Files/Origin Games/The Sims 2 Ultimate Collection. If you open any of the folders in your download, you'll see they contain a folder called TSData, then one called Res, then Lights. You'll want to only copy the Lights folder, because a lot of the folder names aren't completely accurate anymore. Now, first open your Apartment Life folder (the one on Program Files). Go to TSData/Res. There's a folder called Lights. Rename it (I went with Lights_AL_old so that I could recognize it later) and move it somewhere else. Then copy the downloaded Lights folder.

This is the easiest part. Repeat this step for Bon Voyage, FreeTime and Seasons. The others will require a step more.

If you open the Best Of Business folder, you'll see that there's several things stuffed in there. For now, you'll want to open the folder that says EP3. Now just repeat the steps from before. Do the same for Fun With Pets and University Life, always picking the folder named EPX.

Next, check your download for the folder _Deluxe Editions. You'll want Double Deluxe. This one's already prepared, so you can copy the Lights folders from all three download folders like normal.

Okay, almost done here. We've changed the base game and expansion pack files, all that remains is the stuff packs. That is the real tricky part, because they're not numbered. What is numbered are the files in the Lights folder, which is wonderful. So it goes like this: Open downloaded Lights folder, look for filename SPXLights.nlo, search your files for the corresponding Stuff Pack, do the replace shuffle. Since I'm doing this right now anyway, have a list for convenience:

Celebration! Stuff - SP4 - located in Double Deluxe, meaning this has already been replaced. Ignore.
Family Fun Stuff - SP1 - located in Fun With Pets
Glamour Life Stuff - SP2 - located in Glamour Life Stuff because things were not confusing enough already
H&M Fashion Stuff - SP4 - Wait... Okay, so apparently this is the fifth Stuff pack, but the file is named SP4. - SP5 - located in Best of Business - Why?
Ikea - SP8 - located in University Life
Kitchen&Bath - SP7 - located in Best of Business
Mansion&Garden - now the thing is called EP9 but SP9 fits - located in Fun With Pets
Teen Style - SP6 - located in University Life

And that takes care of the Stuff packs.

There are some optional files remaining, check the readme to see if you want them. I hope this made sense to you, ask me if there's anything left that's unclear.