It's not clear if this is meant to replace The Brady Center's "God Not Guns Sabbath," which has been observed on the last weekend of September for a number of years. But the organizers seem eager to keep the event broadly ecumenical and interfaith. So if you look through the resources provided, you'll find things for Jewish congregations and a variety of Christian traditions. There are even some secular statements and a quote from Thich Nhat Hahn that would be right at home in a Unitarian-Universalist worship service. I didn't see anything geared specifically to Muslims, but perhaps I overlooked it.
This broad approach seems sensible for an event aimed at large participation. But I do have to wonder if the organizers aren't missing an opportunity rooted in the specificity of Christian worship. It seems to me that eucharistic worship offers exactly the kind of witness needed in this situation. I am following here the thought of William Cavanaugh, who finds in the eucharist a "different imagination" and a "dangerous memory" that allows a new society to be built right under the noses of principalities and powers:
In the body of Christ both pain and joy are communicable. “When one member suffers, all suffer together with it; if one member is honored, all rejoice together with it” (I Cor. 12:26). In this reality of shared pain, we see the distance between friend and enemy overcome. For the sharing of pain goes beyond a sharing with other members of the Church. If the Church is the Body of Christ, the sacrament and sacrifice for the world, then we are to be broken and given away as food for others. The Church is, as Paul says, to “make up what is lacking in Christ’s afflictions” (Col. 1:24), by suffering together with the victims of violence. If it is the case that the Eucharist makes the Body of Christ, then the Church does not simply commemorate God’s “no” to violence, but embodies God’s answer to violence in the world. We ourselves prefer to absorb the violence of the world rather than to perpetrate violence.
Cavanaugh writes about the Sebastian Acevedo Movement Against Torture, which took to the streets of Chile to celebrate communion as a challenge to Pinochet's repressive regime. People met at the Lord's table in public to proclaim peace and reconciliation—a paradoxical reality in the violent death of just another young man living in just another violent, unequal society not all that different from our own.
I see no reason why anti-gun-violence activists couldn't do the same thing.