There are lots of reasons why people might oppose gun control. As with anything, there's not usually a single reason or set of reasons that make someone support something.
In America in particular there's the constitution. Some are VERY protective of the constitution. If one constitutional right is taken away (the right to bear arms) then the logic follows that other constitional rights can be taken away too - freedom of the press, the right to petition the government and so on.
There are the people who believe that government should govern FOR the people, but not have unlimited power over them. A well-armed citizenry is a practical deterrent to any leader who decides to dispense with democracy and appoint themselves president for life. It would be more difficult for Biden to decide to enforce a dictatorial regime in the US than it would be for Johnson to do in the UK. Assuming they both have the support of the armed forces, in the UK a citizen uprising would mostly be armed with knives and garden tools at most. In the US the public would be armed with, if not military grade weapons, certainly something closer to that standard.
Another pro-gun argument is that guns are just tools. They have lots of legitimate uses (hunting, vermin control, sport, defence, law enforcement) but can also be used for illegal purposes (robberies, school shootings etc). But this is true of lots of things. We don't ban cars because terrorists drive them into crowds. We don't ban kitchen knives because terrorists use them to kill. We don't ban nuts and bolts because terrorists use them in shrapnel bombs.
Think of the situation in Ukraine and imagine a similar invasion happening to the US or UK. Who would fare better? Undoubtedly the US, because many of them are armed. Would/how could the UK quickly arm people and train them to use firearms against an invader? It might be possible but I doubt the government is hoarding millions of weapons (they weren't hoarding enough PPE at the start of Covid, and that's a darned sight cheaper).
Schools in the US are convenient targets for mass shootings. The very nature of the location means that the majority are unarmed (many schools ban weapons altogether) and also untrained. The pro-gun logic (which is not without merit) is that having "good" people with guns on site limits the danger posed by "bad" people who come in with guns. I say this has some merit because it's true; we've all see then episode of 1000 Ways to Die where a would-be bank robber accidentally walks into the shop next door. A shop which happened to be a gun shop, with staff and customers who were armed and able to neutralise the threat in a swift and permanent manner.
My view is that maybe the US is too lenient. The UK is definitely too strict. Given how shootings still take place here, the logic that restricting firearms prevents shootings is clearly false. There's no way the Plymouth murderer would have been able to kill as many people if members of the public had been armed, for instance.