Can anybody help me How to change the datetime field from any other timezone(Which is coming from a picklist) to GMT. My exact requirement is change the datetime field  to a timezone which is coming from the picklist and again converting into GMT. 

My code is below here

DateTime CurentTime = system.now();
String TimeZones = '(GMT+05:30) India Standard Time (Asia/Kolkata)';
List<String> lststrsplit = TimeZones.substring(12,TimeZones.length()).split('\\(',2);
        string strTimeZone = lststrsplit[1].substring(0,lststrsplit[1].length()-1);
        system.debug(CurentTime+'abc#');
        string strCurrentTime = CurentTime.format('YYYY-MM-dd HH:mm:ss', strTimeZone);
        Datetime dateCurrentTime = Datetime.valueof(strCurrentTime);
        
        string strGMTTimeZone = 'GMT';
        string strGMTtime = dateCurrentTime.format('MM-dd-YYYY HH:mm:ss', strGMTTimeZone);
        Datetime DateGMtTime = Datetime.valueofGMT(strCurrentTime);
system.debug( strCurrentTime);