Why are outer planets gaseous in nature whereas inner planets in our solar system are rocky?
Formation of the Solar System.
Our Sun was formed by a rotating cloud of dust and gas called a Nebula mainly due to a nearby gravitational event that collapsed the nebula, more and more matter began to accumulate at the center making it hot enough to support nuclear fusion.
At first the Sun was much more radiant and the Solar system was filled with gas mainly hydrogen. The radiating Sun managed to radiate out that gas from the inner Solar system leaving only the rocky cores which are now also called terrestrial planets.
Sun was radiant but was still not radiant enough to draw that gas out of the outer Solar system and with the passage of time those rocky cores managed to accumulate mantles of gases which are now called gas giants.