Hi, I am making some very basic tweaks to Biomes O' Plenty. The tweak is using IFuelHandler to make certain items usable as a furnace fuel, which currently works without errors. And I know this is probably a noob question, but I get a compiler error when trying to build the mod. In order to be able to import stuff from this (non-developer since the mod doesn't really offer that) class, I added the universal.jar of the mod and added it to the "Referenced Libraries" section of Eclipse. I can use the mod in eclipse, It doesn't get errors or anything when I try to use the blocks, and it all works logically. I just have 0 experience building on top of another mod and or trying to use its class files as an API. No dev jar is available for BOP to my knowledge, but some of the package names have ...API... in them.

Eclipse isn't used to build your mod Gradle is, and right now, Gradle doesn't know about the Biomes o' Plenty jar. You need to add the BoP jar to the libs folder in your mod folder. After that, Gradle should automatically pick up the dependency.