Heat oven to 400°F. Cut the top from the garlic head, drizzle with 1 tablespoon oil and wrap in foil; roast for 35 to 45 minutes until the cloves are soft enough to squeeze.
Toss tomato halves with remaining oil, salt and pepper on a small rimmed pan. Roast alongside the garlic for 25 to 30 minutes until wrinkled and lightly browned.
Melt butter in a saucepan over medium-low heat and stir in flour for 1 minute. Whisk in milk gradually, simmer for 3 to 4 minutes until lightly thickened, then squeeze in the roasted garlic and mash or blend smooth.
Remove the saucepan from direct heat and stir in Gouda until melted. Keep warm without boiling.
Warm broth in a wide skillet, add roast beef and turn gently over low heat for 3 to 5 minutes until the meat reaches 165°F. Lift out with tongs, leaving the broth behind.
Split the rolls and toast only the cut faces under the broiler for 1 to 2 minutes. Divide warm beef among them, spoon on garlic-Gouda sauce and add roasted tomatoes with their loose juices left in the pan.
Close, rest for 2 minutes and cut across the middle. Serve one sub per person while the sauce is warm and soft.