How do I beat gold rank strike team missions?

My strike team members fail a lot, even on bronze. Their success chance on gold rank missions are way way low (15% or less), but those are the only missions I have left.

How do I complete them with my strike team?

If your team doesn't have enough effectiveness, then they simply don't have enough effectiveness. They'll need to level up more, by completing more strike missions. To avoid gaining negative traits, you shouldn't send them on missions they have little to no chance of finishing successfully.

If the only ones you have left are ones they can't do, then it's best to just wait for new ones.

I have a level 9 team that usually has about a 60% chance to complete silver missions, but essentially no chance to complete gold missions.

According to the Gamepedia wiki,

Once you've maxed a squad's level, equipment is how you will give them the additional boost to succeed on Gold Tier Strike Missions. Even at the max level of 20, squads have an average success rating on gold tier missions of only 50-55%. Equipment is just like your weapons, or abilities. Once purchased, they provide a permanent boost to your squad's effectiveness. They retain the equipment from mission to mission, pass or fail.

So, in order to consistently beat gold missions with strike teams, you'll have to max their level and give them lots of equipment.