First thing you need to know is this only works for games with a workshop set to unrestricted. This is a developer setting somewhere, and if it's not set then you cannot download the mod without owning the game on Steam. Luckily, Kenshi's devs were kind enough to do so.

First step is to download SteamCMD, it's a developer tool provided by Valve for running commands on the command line. You can download it from Valve here: https://developer.valvesoftware.com/wiki/SteamCMD

Once you've extracted the file, run it and wait for it to finish any updates it needs. Once it's done you'll be left with a line that says:

Next you'll need to login. You can use your actual account information if you want to for some reason, but I don't suggest it. You would after all be entering your account details in plain text for the world to see. Instead I suggest logging in anonymously with this command:

Once it has logged you in, you'll be met with the Steam> line again.

Now you'll need to grab some ID numbers.

The first number is the AppID, in this case that would be Kenshi's ID. You can easily find it on the game's store page, in the URL. For kenshi the URL is https://steamhost.cn/app/233860/Kenshi/

So "233860" is our AppID.

The second number is the workshop item's ID. You can find this in the URL of the item on the workshop. In this example I will use the 256 Recruitment Limit mod, which has the URL https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=705119823.

So our ID in this example is "705119823"


Once you have your IDs, return to SteamCMD and type the following command:

Which for our example would be:

Assuming the workshop is unrestricted, and you got the correct IDs, it will download it shortly, and tell you the download folder in the success message. You may download another mod after this by running the command again with the new ID.

Once you're done, close SteamCMD with the command:

From here you'll need to move the downloaded mods over to your game's local mods folder. If you got the game on GOG and installed through Galaxy it will be something like:
C:\Program Files (x86)\GOG Galaxy\Games\Kenshi\mods

Otherwise, it will be wherever you installed the game.

You will also need to rename the folder for each mod, as they will initially just be named after their ID numbers. To be read by the game they need to match the mod file inside the folder exactly. For our example mod, we would need to rename the folder from "705119823" to
"256 Squad Limit"