Breed GL to GL until you see something you'd like to change. Even by doing so it does not guarantee that you will see the change. I am keeping a black line, just in case I see something that I might work on, but looking over this year's chicks, there are 3 pullets that I will breed next year. GL x GL. No black needed.
You might have exactly what you need right in those eggs that you won.
One reason to cross to black would be to add genetics that you don't have. But in my opinion, if you jump into that without knowing the negatives and positives of the new line, you might fix one thing and create two that you're not happy with.
My advice would be to breed what you have first. Observe them for 6 months and then decide.
Keep in mind, that crossing to black will create black chicks for 2-3 generations. Only around the 3-4 generation will you see the complete lacing. If you are planning on selling GL, it will have to wait until the lacing is complete and they are breeding true.