Geodude in Pokemon GO just finished up a Spotlight Hour on Aug. 25. If you still have yet to catch the Rock and Ground Type, or its Rock/Electric type counterpart Alolan Geodude, there's a couple things you can do to maximize your chances.
Geodude Pokemon GO: How to Catch
Geodude is most likely to spawn in partly cloudy and sunny weather, whereas Alolan Geodude is more likely to spawn in partly cloudy and rainy weather. This is because of the weather boost mechanic which boosts moves of certain types and spawn rates of Pokemon of certain types depending on the weather.
Rock and ground type Pokemon are boosted in partly cloudy and sunny weather respectively, whereas Electric types are boosted in rainy weather. Geodude is also known to hatch from 2km eggs and can be found more frequently around "rocky" areas like quarries and parking garages. Just be careful you aren't trespassing if you decide to check out a location looking for one.
Another Kanto region Pokemon will be able to be caught more frequently during the upcoming September community day. Be sure to look for increased spawns of Porygon out in the wild when the event rolls around.