Install the plugin by downloading the gplugin file and dragging it in to Geneious or use the plugin manager in Geneious (under Tools - Plugins in the menu).
Once installed, run the plugin by selecting a multiple sequence alignment and clicking on Tree in the toolbar. Garli is available as an option at the top of the Tree building options window.
Zwickl, D. J., 2006. Genetic algorithm approaches for the phylogenetic analysis of large
biological sequence datasets under the maximum likelihood criterion. Ph.D. dissertation,
The University of Texas at Austin.