Adding this information on a page level is maybe not the best approach because if you have several pages looking at the G/L entries you will have to extend all those pages. So instead you should do a table extension on the G/L Entry table.
But if you really want to do it in a page you should use the OnAfterGetRecord trigger. Then use the source type and source no. to look up the information for either vendor or customer. But again - i advice you not to follow this approach.
This is a request we frequently see from new BC users but after a while when they get used to looking up related info in the system it usually blows over.
