Hi,
In Lync Server 2013, you configure Gallery View video conferencing in conferencing policy. Gallery View is turned on by default. If you do not want to allow Gallery View, or want to allow it for only some users, you need to turn off the feature in conferencing
policy.
Configure Gallery View with the following conferencing policy settings:
AllowMultiview
This parameter controls whether a user is allowed to organize Gallery View video conferences. This parameter applies to scheduled and ad-hoc meetings created by the user.
EnableMultiviewJoin This parameter controls whether a participant in a meeting receives the Gallery View video experience in meetings that allow it. This parameter controls the user's
experience in any meeting in which he or she participates.
More details:
https://technet.microsoft.com/en-us/library/jj204871.aspx
Best Regards,
Eason Huang