Bring celery, sugar, celery seeds, and 2 cups water to a boil in a medium saucepan. Reduce heat and simmer until celery is soft and liquid is reduced by half, 30–35 minutes. Strain through a fine-mesh sieve into a small bowl, pressing on solids. Strain celery syrup again, if desired, through cheesecloth-lined sieve (to make liquid clearer). DO AHEAD: Syrup can be made 2 weeks ahead. Cover and chill. Stir gin, lemon juice, and 1 cup celery syrup in a large pitcher. Divide among glasses filled with ice and top off with seltzer. Garnish with celery stalks and lemon wedges.