There are two thing to make it visible and able to change. The gradient component needs to have points on it, similar to a slider, so that it can change as you move the slider. And the preview cylinder needs to be checked so you can see…"
I was a 3 year user of GC - fantastic program but it was just too buggy, non-intuitive, no community and not documented at all so switched to GH. I was RhinoScripting before which is probably easier than VB as a lot of the methods have already been written but... it's not parametric which is a blow to the feedback of generative design.
To start with VB I would go through the Grasshopper Primer v2 - it's got quite a lot of geometry methods explained. Start with generating point collections through loops, then creating nurbs curves, nurbs surfaces, brep, intersections, projections,.... This should give you a handle on the basic operation of scripting.
From here, start building up a library of VB scripts (with attached images) - I find this really helps as you can pull parts of scripts to append to other scripts - rarely I will write a new script from scratch. I keep the scripts (+images) in folders relating to their operations (eg. point collections, fields, utilities, etc.).
You could then start looking at further pushing scripts in the way you handle data - arrays vs lists; subs; functions; case statements; etc.
There's A LOT not documented with GH VB scripting so you can either look to the Rhino SDK documentation on the Rhino site or search the web for general VB methods.