What is the best way of making sure that my edited super-duper new modified Advantage and Disadvantage list is loaded over the default GURPS one?
A. The best way is to create a new datasheet (*.cds file), go to “File -> Load data sheet…” and in that window, select it in the “Data Sheet Files” list and hit the “Load Always” button (the one below the list your new file is in – there are separate buttons for the “Data Sheet Files” and “Loaded Data Sheets” lists). That will ensure that your new data sheet is loaded. However, note that unlike some of the other GURPS character generators out there, duplicate items with the same name from different data sheets will not ordinarily replace each other – they’ll both be listed. However, GCB items have a “priority” property to allow replacement of items.