How to make γ-MnO2 in a lab?
A quick search yields this. 


  [Electrolytic manganese dioxide] is produced in a similar fashion as
  electrolytic tough pitch (ETP) copper: The manganese dioxide is
  dissolved in sulfuric acid (sometimes mixed with manganese sulfate)
  and subjected to a current between two electrodes. The $\ce{MnO2}$ dissolves,
  enters solution as the sulfate, and is deposited on the anode.


Unfortunately, there is no source for the information.

Some further searching yields this article, with the following information:


  EMD ($\gamma$-$\ce{MnO2}$) can be prepared from the direct electrolysis
  of an aqueous bath of manganese sulfate and sulphuric acid. The
  electrochemical deposition of $\ce{MnO2}$ on an inert electrode from
  an electrolyte containing $\ce{Mn^2+}$ occurs as per the reaction
  noted below:
  
  $\begin{alignat*}{3}&amp;\textrm{anode: }\quad&amp;&amp;\ce{Mn^2+ + 2H2O} &amp;&amp;\ce{-&gt;
MnO2 + 4H+ + 2e-} \\ &amp;\textrm{cathode: } \quad&amp;&amp;\ce{2H+ + 2e-}
&amp;&amp;\ce{-&gt;H2} \end{alignat*}$


Note that the linked article is a review, with a lot of information, both about the actual structure of $\gamma$-$\ce{MnO2}$ as well as the reactions that lead to the production of the correct crystalline structure in the presence of acid. There are a number of references that you may find useful as well.