Hi,
Unfortunately there isn't an option to specify your own values to symbolize by - niether of the rendering options quite fit what you want to do. Using the unique values renderer you would have to symbolize each ID individually and using the classify by color creates groupings automatically (based on some choices you make).
I would suggest adding this information to the gpx file directly. Open it up in a text editor then add the information to each waypoint, copying the structure of other attributes. For example for waypoints 1-17 add "Day 1" for 18-30 add "Day 2" etc... After you have re-imported the data, just symbolize by unique values on this new field.
Hope that helps
mike