Getting Married in Gibraltar, Do you need a certain visa?

US citizen traveling to UK to visit (visit is under 2 weeks) my boyfriend and spend time with his family. Planning to fly to Gibraltar towards the end of my visit and get married and then fly back and head back to US. Do I need a marriage visitor visa or any sort of visa to do this? What should we say the purpose of my visit is when I enter UK? Originally we were going to get a marriage visitor visa and get married in UK, but Gibraltar seems easier and less stressful.