Can I use in comparison with and compared to with gerunds? I tried to find the answer in WordReference Dictionary, but it only says that they can be used with objects, and nothing about gerunds.
It's got nothing to do with the gerund. Your sentence ("Learning languages is far more interesting in comparison with/compared to sweeping floors, covered by tonnes of dust.") is wrong because in comparison to and compared to don't use a comparative. You use than with a comparative You say:
Learning languages is far more interesting than sweeping floors, covered by tonnes of dust.
Learning languages is interesting in comparison with sweeping floors, covered by tonnes of dust.
Learning languages is interesting compared to sweeping floors, covered by tonnes of dust.