Cucumber, while to be treated carefully, can actually be a very useful food in hot weather. A small slice in summer weather can help avoid dehydration. But you must be careful with it, as not only can it cause gas, but also diarrhea in excess.
I second Bpatters, any dark, coarse green veggie like kale, spinach, cabbage, can lead to gas. They are high in vit c though, so a little bit every so often will be beneficial to your herd. But certainly not every day. My pigs go through phases, some weeks they love the stuff and some days it's abandoned in the bowl
If you're looking to add variety, radicchio, curly purple and green lettuce, corn husks (great for teeth and can usually be gotten for free from your local grocer), and the odd bit of dill are very popular with our girls.
