I am a plumber who has to travel to various jobsites at my own expense.Can I write off my gas for taxes?

Absolutely!! Save your receipts!

You can also start a spreadsheet with your dates and costs (long as you have receipts), so that it's not a monster to manage at tax time.

Yes, also wear and tear on your vehicle, as long as you're not doing personal thing on the way.

You can write off vehicle depreciation, insurance,(if you can prove that vehicle is only used for work) repairs, and maintenance. If vehicle is dual use( work and personal) you can write off a percentage determined according to your mileage log.