I could not find any information on how the GPU module will be connected. Is it a proprietary connector or a standard 4/8/16x PCI slot which would be compatible with any PCI card? Will other modules such as a battery, SATA SSD, or network expansion cards be possible? Will any of these be available on launch?
I encourage you to read through some of the threads on speculation of what will be possible in the bay. Cliff’s notes of it is: it uses a custom 148 pin connection, with 8 lanes of PCIe Gen4, and 4 lanes of DisplayPort, as well as bidirection power delivery around 200W each way. So a battery is possible, as well as pretty much anything you could run off of PCIe. The full pinout and spec of the connector is available on Framework’s GitHub here.