Do cards like Giant Growth create a one-shot or continuous effect?
Not quite correct. It is just a simple continuous effect.


  611.1. A continuous effect modifies characteristics of objects, modifies control of objects, or affects 
  players or the rules of the game, for a fixed or indefinite period. 
  
  611.2. A continuous effect may be generated by the resolution of a spell or ability. 


So Giant Grown resolving generates a continuous effect. No one-shot effect happens:


  610.1. A one-shot effect does something just once and doesn’t have a duration. Examples include dealing damage, destruction a permanent, putting a token onto the battlefield, and moving an object from one zone to another.


"Getting +3/+3" is a continuous effect, not something that just happens at one moment in time. This can also be seen in the examples listed under the Continuous Effects section of the rules:


  611.2c ... Example: An effect that reads "All white creatures get +1/+1 until end of turn"...