Using Moodle 1.9.11 (Build: 20110221)
Cannot reproduce the problem. Exporting and re-importing GIFT file of category with TRUE/FALSE questions in it works fine.
If you examine the exported GIFT file, are the TRUE / FALSE settings correct (as in see below)? Which language are you using for your questions? Do you have this problem if you create TRUE/FALSE questions in English?
You should try to pinpoint if the problem resides in the export process (i.e. is the GIFT exported file OK) or in the import process.
Joseph
Sample GIFT file:
// question: 0 name: Switch category to $course$/Default for Course Fullname 101/truefalse
$CATEGORY: $course$/Default for Course Fullname 101/truefalse
// question: 732 name: animal
::animal::[html]the pigeon is an animal{TRUE#wrong #correct }
// question: 733 name: flower
::flower::[html]the cat is a flower{FALSE# sorry, cat is not a flower # correct, the cat is not a flower! }