The Gravity Tool/Level Dimension Tool only works for Roof, Shell, Slab, and Mesh.
So what if you want to display the elevation of a sloped beam on each end? You can't do it. There is not even a Label that is useful for showing T.O.B. or B.O.B at both ends of a sloped beam. The work around - if you really want to show these elevations in a Plan View - is to model that sloped beam as a Roof, which is not always a very good option. If it works for Roof, Shell, Slab, and Mesh, why not for at least Beams and Walls too ? Until we have that (probably never) how about a Label that can display T.O.B. and/or B.O.B at both ends? Its not very useful to display wha the bottom elevatoin of the beam is on one end, and the top of a beam at the other end- which is what the current Elevation Lable does. Also, there is the case where sloped beams are taperd from one end to the other.
I frequently would like to place a Level Dimension on a sloped beam for the sake of easily verifying the clearance of something passing under it. There are several things that are useful for the sake of modeling than do not necessarily end up in the Plans. Just some functionality that would be useful sometimes.
Easier for me to do than trying to make Expressions and such, is to just keep a little piece of Roof in the model that I can drag around to where ever I need it to display an elevation at some point in a Plan View. Its not something that is useful very often anyway. Too bad the Gravity Tool can't be used in 3D Documents either.