No they aren't. GMT is not precisely defined, and depending on how it's used can differ by almost 1 second.
UTC on the other hand is not a timezone at all, but is instead a method of defining timezones.
Also, UTC is based on atomic clocks, unlike GMT which is based on the prime meridian. The difference between the two will slowly increase as time goes on as UTC is not bound to the earths rotation like GMT is.
Just because they are often the same doesn't mean they are exactly the same.