请问是gain confidence对,还是acquire confidence对?

两个都是正确的!
acquire confidence 树立信心
gain confidence 获得信心

gain confidence
参考：
http://dict.cn/%E5%AD%A9%E5%AD%90%E4%BB%AC%E5%9C%A8%E8%BE%85%E5%AF%BC%E7%8F%AD%E4%B8%8A%E8%8E%B7%E5%BE%97%E7%9A%84%E8%87%AA%E4%BF%A1%E6%8C%81%E7%BB%AD%E5%88%B0%E4%BB%96%E4%BB%AC%E6%AD%A3%E5%B8%B8%E7%9A%84%E5%AD%A6%E6%A0%A1%E5%AD%A6%E4%B9%A0%E4%B8%AD%E5%8E%BB.htm

两个都对
Have faith in me :)