Jake Gyllenhaal celebrated his 31st birthday with a SoulCycle spin class in Union Square on Monday morning and dinner with friends at Peter Luger in Brooklyn. The “Source Code” star and his actress sister, Maggie, “Love and Other Drugs” co-star Oliver Platt and a handful of close friends shared a low-key meal at the steakhouse. A spy said a “lumberjack chic” Gyllenhaal hung out at the bar and then enjoyed a cake that his mother, Naomi Foner, brought to the restaurant.