What should I put as my GPA for a pass/fail class?
Pass isn't a letter grade at all.  Don't enter anything from the A-F scale; they are all inaccurate.  Check if "P" or "S" (for Satisfactory) are possible options.

Otherwise, reread the instructions for the application.  It may be that they don't want you to include "non-academic" classes like driver education at all.

If you don't find clarification, then contact the school that requires the form and ask them what you should do.

If you are computing your GPA by hand, then pass-fail classes are not included.  Disregard the class completely for the purposes of GPA; leave it out of both the numerator and the denominator.