The GPU inside the processor/soc has the following specifications:
Imagination BXE-4-32 GPU with support for OpenCL 1.2, OpenGL ES 3.2, Vulkan 1.2
Video Decoder – H.265, H.264 4K @ 60fps or 1080p @ 30fps, MJPEG
Video Encoder – H.265/HEVC Encoder, 1080p @ 30fps
I don't think you'll be able to use a separate/external GPU with it. Thunderbolt support is highly unlikely and that processor has only 1 or 2 PCIe lanes (depending how USB is connected), which is likely already used for WiFi.