I agree that "washing" has this implication, but "wash off" does not - it means the same as "rinse off".'Washing' suggests some vigorous motion to get something clean using water and a cleaning product. 'Rinsing' means using water to remove something, the gel in this case, after washing.
There should be some action after putting the gel on, using it, then rinsing off.
Yes, sorry, it was just a silly example I came up with, I didn't think about whether it actually made sense.There's not much difference between wash and rinse but the sentence reads very strangely. If I didn't know how to use shower gel, this would not tell me what to do to get clean.