Follow sharkbait's idea, and remember to change the selection scale and scaling size of the unit, so it blocks the path that you want, because the model might be big, but the collision size might not be there, and the gate/farm might not be big enough to fit the gap.
> What to do?
I'd say give up on the idea and be happy without health being displayed.
If you insist, however, then create another unit with no pathing (use the pathing map for a shrub or something else walkable) and the model of an opened gate. When the unit with the key approaches, remove the gate and replace it with the opened one.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.