You want to install an application on a group of computers using a GPO.
Open the GPMC snap-in.
In the left pane, expand the Forest container, expand the Domains container, browse to the domain of the target GPO, and expand the Group Policy Objects container.
Right-click on the target GPO and select Edit. This will bring up the Group Policy Object Editor.
Under Computer Configuration or User Configuration (depending on which you want to target the installation for), expand Software Settings.
Right-click on Software Installation and select New Package.
Browse to the network share that has the MSI package for the application and click OK.
Select whether you want to Assign the application or Publish it and click OK.
Installing applications with a GPO is a powerful feature, but you must be careful about the network and client impact it can have. If the MSI package you are installing is several megabytes in size, it will take a while for it to download to the client computer, which can result in sluggish performance on the client, especially over a slow connection. You'll also want to make sure you've thoroughly tested the application before deployment. After you've configured the GPO to install it, it will be only a short period of time before it is installed on all targeted your clients. If there is a bug in the application or the installer program is faulty, the impact could be severe to your user base.
Your two options for deploying an application are to assign it or publish it. If you assign an application, it will get automatically installed on the targeted clients. If you publish an application, it will not get automatically installed, but will be available to be installed manually from Add/Remove Programs in the Control Panel on the target computers.