That is because there are no natives named that. Those are AI natives and in order to use them in the trigger script you need a modified native declaration file imported into your map.so im learning a bit of jass but i cannot find the native GetUnitGoldCost in the function list of JassNewgen.. some other natives are missing too like GetUnitBuildTime.
The reason why you have to declare this native in the map header, is because it is not in the function list. The function list contains functions of common.j and Blizzard.j. Some natives that are in common.ai are very useful, but you have to declare them in the map header before using them.
I recommend reading the documentation about native declaration:
JassHelper 0.A.0.0
thank you , thats a lot of natives. still its missing those common.ai natives like getunitgoldcost.