Here's my answer
1. To earn btc, shall I deposit?
That depends on what you are joining. Is it trading? Or Investing? If that's what you mean. Yes, you need to deposit btc in able to earn some btc.
2. Joining bitcointalk.org, how will I be able to earn btc, by joining campaigns?
Yup, but there's a rule you must comply in able for you to get paid from the campaign you applied.
Signature campaign is the highest paying here. But i don't think it suits to you because of your rank. You can do twiiter campaigns if you have good amount of followers.
3. Is gambling a good choice to earn btc?
Maybe... But the chances of being successful is thin. Remember, there's no easy money here and gambling for me is for entertainment only.
4. Are there jobs that pay btc?
Signature campaigns, twitter and facebook campaign, faucets and avatar campaign.
Choose a job that suits you.