Hey there, an easy way to get gradients on curves is to use fit with the point number with a max of npt (total number of points)-1, you can also use the uv texture node set to rows & columns which should produce a 0 to 1 u value gradient.
In terms of lsystems, unless you are customizing it a great deal, I would turn to pops to create your lightning arcs. More control and organic behavior possible there. Here is an example of a 'Rolling lightning' test I did for a job way back when:
For this I had 'arc points' scattered in the volume and a single particle would lead the direction of the arc. As it moved across the point cloud it would send out replicants that would try to connect nearby arc points as well as respect any custom forces (for example I wanted them to go from top to bottom, so they would do a point cloud lookup and get their desired target point then add in whatever custom downwards vector or noise I specified). If this seems like overkill then stick to lsystems, I just wanted to share the concept with you thinking maybe it would get some gears turning
On another note there is a thread: here where a user put forth his space colonization otl that I could see also being very helpful in creating organic arc like shapes. Its a very similar process from what I did only its wrapped up nicely and fairly easy to understand if you take it node by node
-N