How do you carry non-circular drilling with an SMD part in gEDA pcb?
Geda PCB footprints include an outline layer e.g. on a silkscreen layer, if you draw one when you create the footprint. If one of the stock outlines would meet your needs if only it had a footprint, save your own copy of it in e.g. a Footprints folder for your project and make the changes you need.

As regards your oval hole, I would probably substitute a round one drilled to the major diameter, as there is plenty of clearance round it. The part will be a slightly sloppy fit to the PCB as a result, until soldered down. 

Otherwise you could make the oval hole at the PCB routing stage, with a suitably small routing bit (it's an unplated hole) but that's an unusual and possibly expensive processing step, PCB fabs don't like that...

The datasheet shows the slotting is only there because Bourns are a little bit coy about the dimension between the two pegs under the part. So a round hole on the minor diameter (1.55mm) at the right dimension would be OK if you knew the right dimension... 

My suggested major diameter (2.2mm) hole simply allows the part to rotate right or left by 0.3mm before soldering : decide for yourself if this is acceptable on your board.