I found those in in GTWW’s build script and after some hammering (turns out the SNAPSHOT jar no longer exists, but it works with the current version string) the build runs. It’s just that I don’t see the jar file in the referenced libraries.
In particular I added maven { url = "https://gregtech.overminddl1.com/" } to the buildscript - repositories block and compileOnly "com.gregoriust.gregtech:gregtech_1.7.10:6.17.00:dev" to the dependencies block. I have no idea whether that’s correct but it follows the pattern of the other dependency stuff that other people did for me and the GTWW script.