The 35-year-old Haas is the oldest player in the top 100 and is looking for his second title of the year. Haas won another tournament in Munich, also on clay.
Haas cruised through the first set, but Gulbis bounced back in the second. In the final set, Haas broke serve to win the match as Gulbis slammed a forehand into the net.
"There are days like that, I could feel that he couldn't find his rhythm in the first set but I was also struggling. This was my first match after Wimbledon, switching from grass to clay," Haas said.
"In the second, he got some first serves in and they are difficult to return. But I also served well at the end."