Can wire gauge be increased gradually over the length to compensate for voltage drop?
assume 3% acceptable voltage loss, 24Vdc, 10 meter, 50 amp. A cable gauge of 2 AWG 
  would be needed to stay within the acceptable voltage drop.
  
  If the cable length is 10 meter, could the first meter be 10 AWG, and
  the remaining 9 meter 2 AWG?


A given length of wire with some of it a smaller diameter will obviously have higher resistance, and therefore higher voltage drop. But will it be acceptable in this case?   

The resistance of a 10 m run of 2 AWG wire is ~0.000512 &ohm;/m * 20 m = 0.01 &ohm;, * 50 A = 0.512 V, which is 2.1% of 24 V. 

A run with 1 m of 10 AWG wire plus 9 m of 2 AWG wire is (0.00328 &ohm;/m * 2 m) + (0.000512 &ohm;/m * 18 m) = 0.0158 &ohm;, * 50 A = 0.79 V, which is 3.3% of 24 V. 

So no, it would not meet the acceptable loss requirement.