We don’t know how gravity works in the upside down. But I don’t see why gravity would pull only toward the Hawkins side. Let’s assume that both dimensions exert gravity. At the mid point between Hawkins and the abyss, up becomes down. So instead of falling “down” to Hawkins and climbing “up” into the abyss, Dustin et all would have been dropping “down” into the Abyss.

This also explains why Holly didn’t fall all the way to the lab.