Hi

We receive calls from gateway1. Some of the destination numbers are not Lync users. Is it possibe to out these calls to another Gateway ??

EDIT:  It's possible with Lync 2013, but not with Lync 2010.

With Lync 2013, then yes, it's called Inter Trunk routing, and it's mostly configured by adding PSTN usages to your trunks in the voice routing section of the control panel.  Here are a few articles:

http://masteringlync.com/2013/06/07/inter-trunk-routing-deep-dive/

http://ucken.blogspot.com/2013/06/inter-trunk-routing-in-lync-2013.html#comment-form

OK.. a question..

We don't known all destination numbers....so... How Does Lync know to route these calls to other Gateway ??

It depends on how you set it up.  If Lync doesn't have the number assigned to an object, out of the box it will reject with a SIP 404 not found error.  With inter trunk routing configured, and a broad enough PSTN usage in the trunk, instead of rejecting the call, it will send all numbers that aren't explicitly assigned to Lync out to the other gateway for processing.