Gary wrote PL/M for Intel microprocessor 8080 and in the process proved that 8080 was actually a computer and not just a microwave controller! Then to prove that the language PL/M was useful, he wrote the operating system called CP/M. It is what really showed the way for the modern computing. He tried to sell CP/M to Intel in 70s but Intel could see no use for it. DRI also was one of the first companies to develop a GUI called GEM.
He's the man who's famous to have gone flying when IBM came calling. About the creator of CP/M, the story goes that when IBM held a meeting with Gary's company Digital Research Incorporated (earlier InterGalactic Digital Research) to license CP/M, Gary chose to go flying in his plane! As a result, IBM went to Bill Gates' Microsoft and licensed their Q-DOS. Well, this story is only party true. The part about Gary going flying is wrong. As this documentary (Direct download link. Mpeg, 279MB) says, He actually had attended the meeting with the IBM!
About DOS winning over CP/M, what happened after IBM went to Microsoft is that IBM realized that there could be IPR problems with Q-DOS, and hence they went back to DRI for licensing CP/M. This time Gary/DRI licensed CP/M to IBM and also let them sell Q-DOS at the same time in the spirit that the market will decide the winning product. But the marketing strategy of Microsoft was a killer one: While CP/M was priced at $240, Q-DOS was priced only at $40! Rest is history.