How to prepare elemental gallium from a mixture of gallium and aluminum nitrate?
If you have a mixture of aluminum nitrate and gallium nitrate, then you have to separate the aluminum nitrate from the mixture. This patent describe how to separate aluminum nitrate from a mixture of other nitrate and hence, this process can be applicable for separating aluminum nitrate from gallium nitrate.

Then, when you have gallium nitrate, just heat to produce gallium oxide.

$$\ce{4Ga(NO3)3 -&gt; 2Ga2O3 + 12NO2 + 3O2}$$


  The thermal decomposition of nitrate gallium(III) to produce
  gallium(III) oxide, nitrogen dioxide and oxygen. This reaction takes
  place at a temperature of 110-200°C.(chemiday 1)


Then reduce gallium oxide to gallium using some reducing agent like carbon or hydrogen.

$$\ce{Ga2O3 + 3H2 -&gt; 2Ga + 3H2O}$$


  Gallium(III) oxide react with hydrogen to produce gallium and water.
  This reaction takes place at a temperature near 700°C.(chemiday 2)


$$\ce{Ga2O3 + 3C -&gt; 2Ga + 3CO}$$


  Gallium(III) oxide react with carbon to produce gallium and carbon
  monoxide. This reaction takes place at a temperature of 850-950°C.(chemiday 3)


You will find this youtube video of help. It shows how to synthesize gallium metal.