I certainly don't believe the gourds are for Purple Martins. I've built a number of P/M bird houses before, and if built correctly, they're occupied annually. That's the BIG IF. There are specific requirements for the size of hole for P/M's. Too large and they're bird houses are invaded with sparrows, etc. Too small and they can't get in. I think the hole size is 1 5/8". And they prefer their bird house to be elevated. 18' - 20' is preferred.
All of the P/M's I've seen eat mosquitoes. Millions of mosquitoes! I've never seen them eating butterflies, but I'm not saying that they don't. They do however, clean up after themselves. They'll clean out their nest before leaving for the winter. And the next February you'll see their scouts coming to check out the facilities before they go back for the hens. If their nest is dirty from other birds leaving a mess, they'll go elsewhere.
I love the sight of a group of P/M's hovering 3' off the ground, snatching up mosquitoes and other bugs. On the other hand, we also have bats here that totally keep the mosquito population subdued.
As another aside, Purple Martins will run off their bitter enemy, crows. They'll gang up on crows and either kill it, or escort it far from their nest. All-in-all, I love P/M's. They're desirable birds to have around, in my opinion. But I've never seen them nest in gourds.