Consider the characteristic curves of a typical small-signal NPN transistor:
Referring to the right-hand graph, we can see that the DC Current Gain hFE is almost constant with collector current provided Vce is maintained constant at 1V. It starts to drop as you approach Ic of 100mA and (not shown) it will also drop at very low collector current.
If you now look at the left-hand graph, the intersection of Vce = 1.0V corresponds to a point on the right-hand graph for each of the base currents from 0.1mA to 1.0mA. As the collector current increases the gain is less, so the space between the evenly spaced base currents becomes less. Now, imagine the transistor is entering saturation, so Vce is less than 1V, approaching zero. For example at 1mA base current, and 0.1V Vce you can see that the collector current is only about 25mA- that means the current gain has dropped to 25 from more than 200. At 0.1V and 0.1mA base current, the gain is more like 50, still a far cry from the 300 at 1V Vce.
As Vce approaches zero, the gain current drops to zero. This makes sense because the transistor needs some voltage to work. At some point very near zero it will even turn slightly negative as increasing base current causes current to flow out of the collector.
The other thing you might find interesting is that at low base current the curves are fairly flat- the gain does not change much with Vce provided it sees "enough".