At home: Whirl garlic, cilantro, oil, tequila, lemon and lime juices, salt, and pepper in a blender or food processor. Transfer to a large resealable plastic bag. When ready to marinate steak (at least 1 and up to 24 hours before cooking), add steak to bag, shake to coat with marinade, reseal, and chill.
At campsite: Prepare a gas or charcoal grill for high heat (450° to 550°; you can hold your hand 5 in. above cooking grate only 2 to 4 seconds). Grill steak over direct heat (cover if using gas), about 4 minutes per side for medium-rare. Let sit 10 minutes, then cut across the grain into 1/2-in.-thick slices. Serve with Four-Herb Chimichurri.
Note: Nutritional analysis is per serving of steak only.