Have you ever used Measurement Studio from National Instruments? This includes Component Works for Visual Basic which makes GPIB programming easy. There are GPIB programming examples in Visual Basic on zone.ni.com that show how to make the calls and there are examples that ship with measuement studio.
You can use language interfaces or the Tools for Visual Basic (ActiveX controls) that are a component of Measurement Studio.
The language interfaces allow you do make calls to the GPIB drivers themselves. The ActiveX controls are what I prefer to use as it's an easier set up mechanism and hides some of the issues with just straight gpib calls to the driver ... a little higher level and easier.
You should be able to find information by visiting the Resource Library in NI Developer Zone: http://zone.ni.com/libraries/ and then by going to GPIB->Programming->Language Interfaces
There are examples there using the ActiveX controls and also with calls to the DLLs.