Can non-Catholics participate in the gestures at Mass, such as kneeling, making the sign of the cross, etc.?
As a congregant, a non-Catholic is allowed to do as much as do Catholics in the pews with the exception of receiving Communion. You can make the sign of the cross, kneel, stand, etc. Those non-Catholics who are guests (as distinguished from RCIA members) can also participate to the extent they feel comfortable doing so, with the exception of receiving Communion. If they are merely there as observers and do not wish to participate, they need only stand when the congregation does and otherwise sit quietly.