To address your original problem about entering "G" for a True/False question: The best way to ensure that students don't enter "G" is to use a popup list whose only options are "True" or "False". See, for example,
http://webwork.maa.org/wiki/PopUpListsLong
for a way to write a list of True/False questions that get shuffled for the students (to remove the shuffling, set $qns = $qn; in the code) and use popup lists for entering answers.
WeBWorK Problems
TF context
This forum has a limit to the number of forum postings you can make in a given time period - this is currently set at 10 posting(s) in 1 day