I see GNU EFI is a tool chain to compile applications, but it doesn't seem to come with many libraries.
Tianocore EDK2 comes with a lot of code, with modules and drivers that I'm still figuring out. By default, it builds a full firmware image that I use as a BIOS.
What I need to do is, develop a network-capable application that will query a server for boot instructions, and then prepare the next steps. The idea is to control the boot process from the management server, so the client should be able to follow the instructions to boot from TFTP, or from local storage, or update local storage etc.
You have two main options for writing UEFI network applications.
Managed Network Protocol has the firmware trying to autoconfigure your network and download files as appropriate via TFTP/PXE or HTTPS.
Simple Network Protocol gives you raw access to a packet interface if you want to handle things yourself.