Heat gas or charcoal grill. In small bowl, mix sugar and Worcestershire sauce until sugar is dissolved. Brush half of mixture over summer squash, zucchini, bell pepper, onion and kielbasa.
When grill is heated, carefully oil grill rack. Place vegetables and kielbasa on gas grill over medium-high heat or on charcoal grill over medium-high coals; cover grill. Cook 6 minutes. Turn vegetables and kielbasa; brush with remaining mixture. Cook on open grill 4 to 6 minutes longer or until vegetables are tender and kielbasa is hot.
To heat pita bread halves, wrap in foil; place on grill 3 to 5 minutes or until warm.
Remove vegetables and kielbasa from grill. Cut into bite-size pieces; place in serving bowl. Add oil; toss to mix. Fill warm pita bread halves evenly with vegetables and kielbasa.