My pleasure to help, and I sincerely hope all turns out well. In terms of it being hazardous, there’s little reason to suspect that’s the case if this hole/pipe has been there all along. You might wanna check for carbon monoxide, which is something you always wanna keep an eye on. Presumably you don’t smell anything. But old fittings are very common. They’re rarely removed by DIY homeowners or contractors, mainly to keep costs down. For fun, you might wanna do a little snooping in a crawl space or attic, if there’s any such access available. Start with your gas line theory, which sounds reasonable. The pipe should be tarnished copper, or maybe GSP (galvanized steel pipe). If the building was fitted with gas, you should see gas lines of identical material in other utility areas, like the laundry room, kitchen, bathroom, hot water heater and HVAC. Look under sinks and inside closets. Gas lines will typically follow plumbing lines due the “chases” required, which are the accommodating spaces built into the architecture, but not always. Ask an older neighbor who owns a house/unit built at about the same time as yours. You’re first instinct - to be cautious- was your best. It’d be best to identify it, if practicable. Again, the forensics can be fun, and you’ll likely develop useful information when other projects come up. 🚀