Pumpkins

Cant see why not - but they do need a lot of room -Pumpkins are a warm season crop that need good soil, a lot of sun, and plenty of room to grow. Pumpkins are frost sensitive, but fast growing so they do well even in short growing season areas.
Pumpkins need fertile, aerated soil that is warm enough for germination (70°-90°) and warm enough for growing (65°-75°). Mix compost or well-rotted manure into the bed to ensure plenty of nutrients. Pumpkins grow best in soils with a pH between 5.5 and 6.5. Pumpkins need moderate amounts of potassium and phosphorus and high amounts of nitrogen. Pumpkins need to be watered regularly throughout the growing season. Keep the pumpkins evenly moist and water deeply during dry spells. To prevent mildew, water pumpkin plants at their base, don't water the pumpkin foliage. Watering the pumpkins in the early morning also helps prevent mildew.

Pumpkins can have problems with the humidity of a greenhouse unless it is very well ventilated. You have time for an outdoor crop, and would probably get larger and more production.