To gain respect, stay in your neighborhood and get a wanted star, then kill all the police that arrives. Once you reach a four star wanted level (and if you killed enough police), you will gain some respect. Additionally, completing missions is not the only way to earn respect. By spraying other gang's tags, you will also earn a bit, and by taking over another gang's area you will earn a lot. You must have completed some missions before you can take over areas. You can see on the radar witch areas you have taken over, and which areas the enemy gangs own. Your own territory is marked with green, and your enemies are yellow or purple.