You have 2 options as far as the gas leak goes. One being to contact the gas company. They will send someone out to check it out. The second, which if I was you I would do because it’s going to get someone’s attention is to call 911. As a volunteer firefighter I know what there response is going to be, or at least should be. They will send out a few engines, a few cops, and a few ambulances. They will evacuate the building and the surrounding buildings and block off the area. This will get someone’s attention.
As far as the mail goes. Like a few others said, contact your post office. Fucking with mail is a federal offence. The company maybe the one who has to pay for the mail box but under federal law technically the post office owns it.
As far as the bug problem you could look into contacting someone from the city. Not really sure who but my guess would be the building inspector. Most cities will go after slum lords.
Make sure you document everything so incase you do have to go to court over this you have the proof.
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