In criminal circles a golden tooth could serve a decent payment. So, the phrase "[otherwise] I give [you my golden] tooth" means the speaker is so sure in something that he is ready to give up his golden tooth in case that something would not work out. That phrase is usually accompanied with a gesture in which the speaker opens his mouth demonstrating his golden tooth and puts his thumb behind the tooth promising he would take it out. So, the meaning of "зуб даю" is that something is a sure deal.