First – Why GTD doesn't work for intellectual work
GTD is all about stress free productivity, it works by creating a system you can trust with commitments and deadlines, so you can let things fall out of working memory without getting dropped
You can't put a deadline on insight though. Making real progress on an intellectual problem requires drawing connections between things you hadn't connected before.
Can't predict when you'll get the idea that ties everything together or realize you were asking the wrong question