GEX is a general purpose input/output device that attaches to a PC or laptop. The module, controlled by a Python or C program running on the host PC, provides easy access to on-chip peripherals and low level functions implemented by the firmware.
The platform is designed for quick hardware prototyping, tinkering, automation, and measurements. In addition to custom hardware, the firmware can run on STM32 development boards, which makes it accessible to everyone. Even better, a version for the eBay-popular STM32F103 Bluepill board is planned.
You can try GEX right now, perhaps with one of the pre-built firmware images, but keep in mind that it is still under development and despite having version 1.0.0, it is closer to beta than a stable release. In particular, the API may change in backward-incompatible ways (obviously accompanied by a minor version bump).