You cannot. If it were possible, it would be everywhere and people would be comparing it to Vision of Confluence. Plus, the perk pools are small so it would only take 5-6 rerolls on average to get Full Auto so I would've almost certainly seen it... there are players who've spent 25+ rolls trying to get it but I've never seen one with it. Gheleon's Demise seems to have the same perk pools as all the other Legendary Scout Rifles, which lack Full Auto. It's one of the things that makes VoC special.