It's like saying, little Timmy is 1 meter tall. Does that mean that Timmy's mom is 1.6 Timmies tall? Technically she is, but it's better practice to measure Timmy's family members in meters instead of Timmies, and say his mom is 1.6 meters tall.

GMT just happens to be UTC+0. They are "the same time", but one of them is the actual measurement tool (UTC), the other is the measurement value as measured by that tool (GMT); in the same way as a meter stick is the tool, and the 1-meter tall Timmy is the measured value.