I believe when people refer to a boil gravity they mean a pre-boil gravity check to see if they have hit a pre-boil calculated gravity after a sparge. The original gravity would refer to the post boil gravity that is being transfered to the primary. Again that's how I view the boil gravity.
What LS Grimmy said. From your post you are interested in knowing your pre-boil gravity from which you can predict your OG when the boil is done once you know your boil off rate. This allows you to adjust your gravity or hop additions in order to hit your BU:GU ratio in the final beer.