Cathode gained mass in zinc electrolysis
If the differences between the two electrode masses is small its probably due to excess electrolyte left on it or a measuring error. The white substance is  dried up zinc nitrate. 

Bubbles should have been observed at both electrodes since that would have been the hydrogen, oxygen and nitrous oxide dissociating form the diluted solution of zinc nitrate. 

ie: $$\ce{3e¯ + 4H+ + NO3- -&gt; NO + 2H2O}$$ $$\ce{O2(g) + 4H+(aq) + 4e -&gt;  2 H2O}$$

So I'm pretty sure the bubbles didn't have anything to do with the additional mass gain. $%edit$