7. Astro — 320 hpOne of the oldest GMC trucks on this list, the Astro line ended production by 1987. However, even with its notable age, this cabover managed 320 hp. Again, this doesn't mean much in the modern era, but even a 1989 Corvette would have trouble getting as high as 250. Marketing material pitched the Astro as "ahead of its time," and that may have been true — it just didn't live to see that time come.Interestingly, the Astro name was later used for a line of Chevy vans, lasting up until 2005. As for GMC's original Astro, also called the Astro 95, not a whole lot of information still exists regarding its history or even specs. Its discontinuation in the late '80s was attributed to the slumping sales of the company's heavy-duty lineup, and none of its modern commercial vehicles have attempted to replicate the Astro's design or carry on its name. Perhaps the only thing about the Astro that was "ahead of its time" was its high-power engine, considering the state of cabover trucks today.6. TopKick — 330 hpThe GMC TopKick found a wide range of uses over its active years. Before its discontinuation, its models served as the basis for dozens of commercial vehicles, including school buses and dump trucks. For the average consumer, the C4500 from the early 2000s would provide 300 hp to help it keep up with busy roads and bustling highways. However, multiple TopKicks used for commercial purposes reach as high as 330 thanks to turbocharged engines, letting them do more than just pull their weight.Today, semi-trucks will usually have 400 horsepower or more, but there were few options pre-2010 that could even come close to 300. Even the less powerful version of the TopKick was able to outpace SUVs like the Ford Escape, though it did take a while to get to 60 mph compared to many regular-sized cars. Unfortunately, General Motors filing for bankruptcy led to the shutdown of its medium-duty offerings in 2009, leaving the TopKick in the dust for the foreseeable future.