A 'gummy rub' is when a (usually old) lady or gentleman removes his or her false teeth and gives a (usually younger) gentleman oral pleasure.
This phrase could idealy be used in the following situations -
'What did you get up to last night Charles?'
'ah, not much - me old land lady came round and gave me a gummy rub
... not bad for a pensioner!'
'alright luv, fancy giving me a gummy rub
'I gave this youngster a 'gummy rub' last night but he was really small, not really worth it!'