"Has anyone looked in to how accurate these Gartner figures turn out to be?"
While I'm sure there is a method to Gartner's forecasting, I would strongly suggest that giving the paying customers what they want to hear is a key part of that method.
Which prompts the question, who are the paying customers? Fortunately they usually offer the Gartner reports or at the very least their favourite quotes to the public.
And sometimes, telcos dreams do come true. Whether that is an indicator of accuracy or just dumb luck is another question...