Cook pasta following package directions, about 12 minutes. Add peas to pasta water for last 2 minutes of cook time. Drain and return to pot.
Meanwhile, heat oil in a large nonstick skillet over medium heat. Cook onion and garlic for 3 minutes. Stir in 2 teaspoons Cajun seasoning; cook 1 minute. Stir in shrimp; cook 2 minutes then set aside.
Melt butter in a medium saucepan over medium heat. Whisk in flour and cook, whisking, for 1 to 2 minutes. Slowly whisk in broth and milk. Bring to a simmer; cook 3 minutes. Stir in remaining 2 teaspoons Cajun seasoning and Gruyere until smooth. Pour into pasta pot along with shrimp mixture. Toss to coat; serve immediately.