Melt butter in a saucepan and carefully mix in syrup, eggs and brown sugar. Place soda in a 2-cup measure and pour hot water over it. Mix in butter-sugar mixture.
In a large bowl, combine flour, ginger, cinnamon, cloves, allspice, and nutmeg; stir into batter. Pour batter into greased 13x9x2 inch baking pan and bake at 350 degrees about 20 minutes or until firm.
Top with sauce when cool. In small saucepan over medium heat, melt butter; add rum. Blend in sugar and spread over cool gingerbread.