1. Preheat gas grill to medium-high. Cut kielbasa into 4 pieces and slice each piece in half lengthwise.
2. Split rolls and grill, split sides down, until lightly marked. Remove and set aside.
3. Grill onions, turning once, until charred and softened, about 6 minutes total. Transfer to a bowl and toss with barbecue sauce.
4. Grill sausages, turning once, until hot and lightly charred by grill, 3 to 4 minutes total. Arrange onions and sausages on rolls and serve immediately, passing more barbecue sauce on the side, if desired.