Paraphrase the given lines from the passage:
Imagine then that the gas is somehow disturbed so that one, small, spherical region becomes a little denser than the gas around it so that the small region's gravitational field becomes slightly stronger. It now attracts more matter to it and its gravity increases further, causing it to begin to contract. As it contracts its density increases which increases its gravity even more so that it picks up even more matter and contracts even further.
If the gas moves, a tiny region may become denser than the surrounding gas. Its gravity strengthens and it pulls more matter as it contracts.
When the gas is disturbed, a spherical region becomes denser so that its gravity becomes stronger. This causes it to attract more matter towards itself and become denser.
A small region of gas becomes dense and starts attracting matter, increasing its density and gravity. This continues as it contracts further.
Gravity keeps increasing, as does the size of the gas.