I use an Osprey DC3000 steam machine BUT you will uncover more gum when when you pressure wash tarmac afterwards. I paid a grand for my s/h steamer
I paid 200 quid for my DC3000, borrowed it out and the elements got knackered.
Still got a Vega Plus but even that was playing up overheating a while back.
Just used the hotbox since.
Tarmac, we clean the area first and most gum will appear and stand out then spot the gum. But even low pressure steam can damage tarmac.