Each loot bonus, except by the Gut charm, makes an extra loot set to be generated for you (out of the loot pool and based on the drop rates). The same applies to the Wealth Duplex bonus.
You can easily test it with creatures that have loot with 100% drop rate. When killing such creatures, the amount of extra items that are always dropped will be 1 for each active bonus. However, this will not always be the case for drop rate values different from 100%.
As far as I know, Gut charm adds a 20%-chance of an extra loot set to be generated for creature products, which means for every 5 creatures that you kill, you loot the amount of creature products you would get in 6 creatures (20% bonus), in average.