As far as I am aware, commission is paid by the landlord. The "gross" vs "net" likely means base rental cost with and without "nebenkosten" (not sure what the word is in French). Nebenkosten are the shared or communal costs like cleaning the common area, maintenance on lift, water, etc. that all tenants divide up and pay. What happens is that you will pay a fixed amount of your rent each month towards these costs as an estimate and then once a year there will be a true-up to the actual costs - this can be positive or negative. A word of caution - it is worth it to ask the question what the actual nebenkosten were for the prior year for the particular flat and see if this is reasonably close to what they are quoting as part of the rent - without getting in to a long story, in our case, the landlord significantly underestimated the nebenkosten in marketing the flat and then when it came to true-up to the actual costs we owed 150% more than the amount that had been collected already through the monthly rent.