Why there is no boundary
Earth has a surface because matter changes state abruptly: the air ends and rock begins. Under enormous pressures, this sharp divide does not occur. The gas is compressed more and more until it enters a state that resembles both a gas and a liquid.
Farther down, the pressure rises to millions of times Earth's surface pressure. Under these conditions, hydrogen begins to conduct electricity and behave like a liquid metal. This layer probably creates the planet's powerful magnetic field.
A gas giant has a solid surface beneath its clouds that you could fly down to.
There is no boundary: one state of matter gradually changes into another. There is probably a heavy core at the center, but not a rocky ball with a surface. Instead, it is a diffuse region with matter dissolved in it.
What the descent showed
The only probe to descend into the atmosphere of such a planet was dropped there in the 1990s. It transmitted data until it was destroyed and managed to reach a small depth by planetary standards, detecting stronger-than-expected winds along the way.
One hundred and fifty kilometers may seem deep, but for a planet that could hold a thousand Earths, it is a thin film. Everything deeper is known from calculations and measurements of the gravitational field, not from observations.
By the way, where do we draw the upper boundary? For these planets, the conventional zero altitude is the level where the pressure equals that at Earth's sea level. This is simply a convention for convenient calculations, not anything physically special.
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Sources
- The Galileo probe entered Jupiter's atmosphere in December 1995 and transmitted data for about an hour. The interiors of gas giants are known from models and measurements of their gravitational fields