Download the latest version of the Gallery App checking that the minimum version of Perch is the same or less than the version of Perch that you are running.

Unzip the download where you will find the following folders.

Copy the addons/apps/perch_gallery folder into perch/addons/apps/

If you want to use the field type, copy the addons/fieldtypes/albumlist folder to perch/addons/fieldtypes/

Add perch_gallery to your apps.php file.

Once you have installed the app, log into the Perch admin. Perch will create all the tables needed by the app behind the scenes and you can now start to add albums and images.