the secret is Reshade 4.6.1 (older version from 2014)
In order to be able to use Reshade alongside the Vulkan renderer in gzDoom, you have to download an older version of Reshade, then it will work with Vulkan. Vulkan will allow you to fully use your GPU for bigger Doom maps, avoiding framerate slowdowns and being able to take full advantage of your hardware (something the stock OpenGL renderer still isn't able to achieve)