RetroGradle is an initiative to update the older ForgeGradle toolchains (1.x to 2.3) and the Minecraft versions associated with them, to use the more modern ForgeGradles (3.x and 4.x).
To do that requires a lot of time and scripting, but nothing is "complex".
The update for 1.12 to the FG3 toolchain is the benchmark for how we're going to do the other versions.
The repo update itself can be found here. The largest change here is the build.gradle script. You'll notice that it looks almost identical to the 1.16.x buildscript, with a few minor differences.
Those being, the MCP version (we'll get to that), the run configs using legacydev.Main(Client/Server) rather than userdev.Main, and some other small adjustments to make 1.12 happy.