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What is the make and model of your gun?
I have been looking at schematics and can't really see anything that would cause the gap besides a worn disassembly button or its spring. If the button does not move all of the way out to the "locked" position the slide can be loose.
I would not shoot it with a loose slide, you don't want it flying off when it cycles.
A worn or cracked rail on the slide or frame can cause it to be loose too. Either way I wouldn't trust it.
It may correct it if the problem is in the slide. If it's the frame then a new slide would fit the same, loose!
I don't think they're worth putting money into. A stripped replacement slide is $57 plus shipping. The gun isn't worth much more than that.