In Extensions > Template Manager > rt_crystalline_j16 - Default
Goto the Advanced Tab and check that RTL support is on.
Once its on Gantry should automatically make the site RTL (right to left) when an RTL language is detected.
The RTL setting allows you to enable Right-to-Left support in the gantry core. The Gantry template will be switched to RTL automatically as soon as a RTL language is detected.
crystalline does support RTL so it should work fine if its on, im not sure if it relies on your Joomla default language to be set to arabic, or if it gets the language from your browser?