I cannot find a POI-symbol ‘gas-station’ to place for !?

I am not really sure what you are asking about. Try to be more precise when asking questions.

I am assuming that you are editing with Potlatch 1 ('edit' tab) and want to add a petrol station. There is not room for all the icons on the single page, so just the most used are simple drag and drop. To mark petrol stations double click the map to create a node and then select 'petrol station' from the drop down menu.