Custom mod achievement can not parse - what is "com.google.gson.JsonElement.isJsonPrimitive()"?

Hey there, I am trying to add a datapack for the LevelZ mod and I keep getting errors that point to some "com.google.gson" thing I don't understand. The developer of LevelZ provides an example achievement which works perfectly fine ingame but my custom one won't add for some reason even though it follows the same format. 
Is there some syntax error or some really basic knowledge on datapacks I am missing here? Because I keep trying to make datapacks for a variety of mods and I keep running into issues that point towards this weird com.google.gson invoke. Thanks in advance!