Churches enjoy tax exempt status because of the religious work that they do. From the IRS:
To be tax-exempt under section 501(c)(3) of the Internal Revenue Code, an organization must be organized and operated exclusively for exempt purposes set forth in section 501(c)(3), and none of its earnings may inure to any private shareholder or individual. In addition, it may not be an action organization, i.e., it may not attempt to influence legislation as a substantial part of its activities and it may not participate in any campaign activity for or against political candidates.
A rally where the major focus is the passage of more gun control goes far beyond the scope of a memorial for victims. As soon as you begin a blanket exclusion people of one political opinion and not the other, in my opinion, this is no longer a memorial and is now a political rally, and your church has ceased being a non partisan religious organization and has crossed the line into being a lobbying organization. You want the government to stay out of your church? Then keep your church out of the government.
Any church that uses its self as a shill for any political cause that is not directly related to the church should have its tax exempt status revoked.