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You need to use
pdflatex -shell-escape yourfile
to allow the mp back end.
Also apparently for beamer you need to set the pointsize:
\documentclass{beamer}
\makeatletter
\def\@ptsize{10}
\makeatother
\usepackage[shellescape,latex]{gmp}
\usetheme{Montpellier}
\title{Something}
\date{\toda...