This is a tough concept to pull off, there are several issues that you will need to account for.
Let's start with known natural observations, there is currently no known gas that has a density that is greater than liquid, the densest gas known is still 1/80 the density of water, which would be essential for water to float on it above a solid surface with a standard gravitational pull. Also with each of those layers being kilometers thick the pressure on any gas layer below the liquid layer would be immense forcing the gas to transform into a liquid or solid state.
That being said, when building a fantasy world you are more than welcome to alter any natural laws as long as you can make it believable to your audience.
As for evolution of life in such an environment, i would look to how life currently exists in our oceans, primarily focusing on the ecology of algae life and how it supports biodiversity. Using a similar concept with algae as a basis for the food chain you can build life-forms that get progressively more diverse and complicated the higher they are in the food chain. One other aspect to evolution of a life-form in this type of environment would be what factors would force modifications to its anatomy, being in an all liquid environment for example, would the life form ever have a need to develop legs that could sustain its body weight on dry land?
As for construction materials for a tool-using society in an environment such as this, i would look more towards living structures, something that could be grown or shaped to provide the materials needed. For situations that would need heavy metals, you would still have to solve for a way to either get the materials from either asteroids falling into the water or some volcanic activity forcing material up into the "sea" from below.
Climate is a very complex feature to account for, is your fantasy planet tidal locked? without a major landmass to provide a disruption to a current would the sea follow the gravitational path across the surface of the planet? would the gaseous layer below the "sea" affect the currents of the sea above? Would the major heat source come from a star warming the planet through an atmospheric layer, or would the heat source come from the planet's core, or both?
I would start breaking down your concept into a series of questions and try to answer each one, i know that it will spawn many others as it has for me, but clarifying them will help you to refine your idea into something workable.