I guess they may not technically be necessary, but they sure as hell make it easier. Check out this article for a good take on the issue.
Insulin is the most anabolic hormone in the body. Anabolism is the process where smaller units are made into molecules. For our purposes in training, it means growth and recovery. You can not — repeat, NOT — get optimally strong without it.
and (emphasis mine)
I’m not suggesting you have giant carb meals to create an insulin response. People forget that exercise, particularly exercise that induces a state of growth (like lifting) will increase insulin sensitivity. Fish oil also increases insulin sensitivity, and most people that read this site are taking it. You won’t need crazy amounts of carbohydrates to induce a solid insulin response — unless you’re a Skinny-pain-in-the-ass-Guy. I do want to point out that “crazy amounts of carbs” is a relative thing since a lot of people are eating melons, berries, and pine tree bark as their daily carb source (it’s what Lucy ate, after all). Unless you’re a fat person — and you know if you’re fat or not — aim for a similar amount of carbs and protein.
Fruit and tubers are great carb sources if you're trying to gain mass while staying paleo.
Hate to be a geek here but there is no "how" with necessary. Something either is or is not.
So the answer has to be that no they are not necessary.
However, you will not gain weight efficiently without carbs. And you wont feel as fit or ready to lift hard without them.
Eat your carbohydrates, and eat a lot of them. If your goal is to gain weight then why would you omit a macronutrient?