I was lucky enough to encounter Replete while he was working on this piece. We chatted for a bit, and he kindly showed me his workbook.
Replete's pieces start off as a pencil sketch, then the colour gradients and fancy transparent overlays are done using computer apps. By the time it comes to painting on the gate, he's using his spray-cans to match the digital art. It's complex, and the effect is well worth the effort.