General Motors Co. has unveiled the second in the two-fisted assault it plans to throw at Toyota Motor Corp. this year, as it looks to unseat the Japanese auto maker as king of the midsize pickup truck.

The 2015 GMC Canyon, slated to roll into showrooms in the third quarter, will be paired with the Chevrolet Colorado, as GM returns to a segment it, along with Ford Motors Co. and Chrysler Group LLC, abandoned two years ago. The…