Wow, this coding is shitty. LOL. I would consider asking you to code iItemProbability to read from Settings.cfg instead of HGServer source files, as you can adjust via Settings.cfg on your drop rate instead of keeping changing inside source.
From the codings above, the lower your iItemProbability, the higher % of thing drops. Its default is 8500, which means, the % of drops dropping is only 15%. Which means of every mobs you kill, only 15% chance item will drop, the remaining 85% is no drop.
If somehow if you hit the 15% of drop, Gold will take 65% chance of dropping, and the remaining 35% is items.
As you continue to see below you will see how its being calculated on the items part.