First off you will need to look at your owner controls and click Add/remove required items
After that you will need to select the addon you want it to require. For this case i will use gm_bigcity you can also use ctrl+f to search the addon you want your addon to require more easily.
And after you select the addons you want to be required you will need to press save and Continue And thats it!

this page isnt required for gmod but i'll add it here anyways
To add a DLC to be required you will need to click Add/Remove DLC
and select the dlc thats needed for your addon
Note:only select the required DLC if you used any assets of your addon with that DLC.