Suppose I have a magnet and I put a piece of iron next to it, then the magnet will attract it.
Now if I put a piece of wood in front of the magnet and the piece of iron, the iron will not get attracted. Why?
Now I have heard that gravitation is also magnetism, so it should also show the properties similar to that of magnet but it is not so, even if I put a big building between ground and the object it gets attracted. Why?
There are a couple mistaken assumptions in your questions, which render your questions meaningless.
First, wood has almost no effect on a steady magnetic field. The relative permeability of wood is $1.00000043$, which for most purposes is negligibly different from the relative permeability of air ($1.00000037$) or vacuum ($1$).
Second, saying that "gravitation is also magnetism" isn't really true. Kaluza-Klein theory provides a unification of gravitation and electromagnetism into one theory, but that doesn't mean that gravitation and electromagnetism are the same thing any more than the electromagnetic force and the strong force can be said to be the "same thing" because quantum field theory provides a model which encompasses them both. Gravitoelectromagnetism also points out similarities between gravity and electromagnetism, but gravitoelectromagnetism is only an analogy that's only approximately valid under a limited set of circumstances. Gravitation and magetism are different phenomena.
