Clone into phpBB/ext/gallery/core:
git clone https://github.com/nickvergessen/phpbb3-ext-gallery-core.git phpBB/ext/gallery/core
Enable in database by inserting a row into phpbb_ext
INSERT INTO phpbb_ext (ext_name, ext_active, ext_state) VALUES ('gallery/core', 1, '');
And purging your cache.
##Tests and Continuous Intergration
We use Travis-CI as a continous intergtation server and phpunit for our unit testing. See more information on the phpBB development wiki.