Gun lovers don't usually talk about whether non-Christians are granted rights to have guns by a Christian god, and they certainly don't seem concerned that someone who does not believe in a Christian god might argue that Christians don't have any rights to guns because there is no Christian god, or because some other god or understanding of god says there is no right to guns.
Sometimes gun lovers say that their right to have guns comes from nature, because humans are animals and self-preservation is instinctive. They ignore the larger picture – that social animals naturally protect the entire group, even if the individual must be sacrificed in the effort. They also pay no attention to the differences between humans (which have developed sophisticated technological ways of organizing and defending their complex societies) and other creatures which must rely on whatever tools their genes provide them.
Some gun lovers say that their right to have guns comes from the Second Amendment to the Constitution. After a couple hundred years during which the Supreme Court held a contrary view, the gun lovers finally have a Supreme Court that has said the amendment gives them a limited right to have guns, subject to regulation by the government. The gun lovers accept the part of the court's decision that they like, reject the part about regulation that they don't like, and vow that regardless of what the court says, they insist that there should be no limitation to their Second Amendment rights.
Whatever the gun lovers profess to be the source of their right to have guns, they don't acknowledge that the right could be taken or interpreted away. The Second Amendment could be repealed or a court in the future could disagree with the recent decisions and again hold that the right to bear arms only refers to a well regulated militia, which we don't have and have never had. Even if the Second Amendment remains intact, other parts of the Constitution could be construed as being in conflict with it, permitting severe limitations on private gun ownership.
Gun lovers who think their right to have guns has a divine origin forget that although the U.S. Constitution guarantees that right to freely practice one's religion, there have been plenty of unsuccessful religion-based challenges to laws. It is clear that the U.S. government, its courts included, has the power and the right to enforce its will on its people regardless of what some of those people believe their religion dictates. Everyone has to pay taxes, even people who think god tells them they don't have to.
What gun lovers really mean when they talk about their rights to have guns is that they want to have guns, despite all of the evidence that private ownership of guns has made people in the U.S. much less safe and free than places where there aren't so many guns. The gunners talk of rights, because they think that rights are superior to reason and reality. So long as they can convince people that they have a right to guns, it doesn't matter whether, for the sake of the entire country, they shouldn't be allowed to have them. Rights, to gun lovers, are just a way of getting what they want, regardless of the harm they do to others. The entire American experience has shown that no one has any right under God, nature, or the Constitution, to act in such a manner.