Koshering the Kitchen and Surfaces

The questions concerns koshering the kitchen. I am moving into an apartment where Torah-observant individuals have lived for the past two years. However, before them, two single brothers from a good family lived there; these brothers don’t appear to be Torah-observant. They claim to be vegans. My question is, how do I kosher the kitchen, granite countertops, sinks, and gas stove?

Shalom!
Even with 'vegan' use, there is a problem of surfaces’ absorbing forbidden foods. For example: Shmita (Sabbatical year), Tevel (untithed produce), Orlah (fruit from a tree in its first three years), etc.
The granite countertop: there is no obligation to kosher it, as it is unusual that it be used for boiling water pouring from a primary vessel (that is, a vessel that was heated on a fire or by a heating element).
The sinks: if they are made of stone or metal, you should pour boiling water from a primary vessel over them. If they are ceramic [as most regular sinks are], Hagalah (koshering by boiling water) is not effective. I this case, care should be taken not to use hot water at “Yad Soledes Bo” temperature [approx. 40 degrees Celsius]. However, if hot water was used by mistake, the utensils or food in the sink are not forbidden.
The gas stove: it is sufficient to clean it thoroughly, as the burners become very hot and kosher themselves through use.

Shulchan Aruch, section Orach Chaim, chapter 451.