They really produce a lot of waste. I think the blue print is fine, just not a lot of water even with proper filtration. Like I said it should be fine for a long time maybe even life just be prepared for an upgrade if you can't keep the nitrates down. I grew my gulper out in a 30 but after he hit the 4-5" mark I found the Nitrates were building up to fast even with 50% weekly changes.
I think a gulper being active has a lot to do with how large of an aquarium its in. My Gulper is about 6-7 inches and he is in a 135g aquarium. He swims around just as much as my flowerhorn does. Where when he was in a 45g corner aquarium he wasn't nearly as active.
Look at it this way: A gulper may LIVE in a 30, but would it be anything more than satisfactory? I would err on the safe side and go with a larger set-up. Why cramp it if you can give it more room? If it's a fish that you really want (especially a really expensive one), why take your chances and be cheap with tank size?