1. In a small saucepan, sprinkle gelatin over 1/2 cup water. Let stand until softened, about 5 minutes. Add citrus juice and sugar; cook, stirring, over medium-high heat until steaming, 3 to 5 minutes. Nest pan in a bowl of ice water and stir until cool, then stir in tequila.
2. Line a 4 1/2- by 8 1/2-in. loaf pan with plastic wrap, letting wrap hang over ends. Pour citrus mixture into pan, cover with another piece of plastic (not touching mixture), and chill until firm, at least 2 hours and as long as 1 week.
3. Uncover pan. Holding ends of plastic wrap, lift gelled mixture on wrap onto a cutting board. Cut into 18 squares. Transfer to a plate. If you like, sprinkle with a little salt and garnish with lime wedges.
Note: Nutritional analysis is per serving (3 squares).