With an eye toward growth, The Gateway Engineers Inc. is in the market for a new headquarters and plans to select a new site later this spring. According to CEO Jason Jesso, the firm has narrowed its options to three locations, having culled those from an initial list of 30. He declined to reveal those sites, but said the firm, which employs about 150, is looking for a place that has enough room to accommodate 200 to 250 people. Ideally, he said, the new headquarters will be on a single level,…