This repository contains an example project that uses VGUI outside of the engine. If you wish to make something like this for your mod, it's recommended to apply changes this repository makes to vgui_controls. These include a workaround for Frame rounded corners and a fix for bad border scaling post-HL2 Anniversary.
For this to run natively under Linux, you'll need to implement ISurface for your favorite windowing system, as the game only provides one for Win32GDI and the material system.
Win32 Surface is also not without it's issues, for example transparency doesn't work.
Resources are stored in game\bin\example.
For build instructions see here.
The SDK is licensed to users on a non-commercial basis under the SOURCE 1 SDK LICENSE, which is contained in the LICENSE file in the root of the repository.