Gravatars & photo gallery

Please If you can explain how to use a authors gravatar.
Is it posible to integrate into WP without signing in on gravatar.com?

Can you recomend any gallery plugin for this theme?

There is no way to use gravatar without registering on gravatar service. It uses api to allow multiple serwices to use your once selected and attached to an email address avatar.

As it goest go a gallery plugin - please browse wp extension directory - for sure you will find there something that suits your taste.

Is it posible to turn off gravatars? Or I have to check in php and delete it?

It is WP integrated mechanism and it cannot be disabled easily.
You might try plugin like this:
http://wordpress.org/plugins/disable-user-gravatar/
or similar but I don't guarantee it will work correctly.