decreasing base current on saturated npn transistor
If i decrease the Ib current,will the gain of the transistor will
  arise and keep the same Ic current (20mA)?


No it won't: -



The above shows the relationship between Ic and Vce for various base currents. Each graph is seperate - they do not overlap. If you reduce base current then collector current also reduces.

Some google images show it incorrectly here: -



The mistake here is that all the individual curves are seen as merging and of course that merged part of the graph could imply that if you reduced the base current, the collector current would remain constant.

This would be incorrect!