Ok, here's the problem. SecuROM protected titles utilise an area of a CD called the Subchannels. SecuROM puts some of it's information in there and when the game goes to verify it checks these subchannels for this particular information. Now, there are many formats that can support Subchannels built into it, but unfortunately the ISO format is not one of them. You will have to get your original CD back and re-read from it again with a program with Subchannel storing capabilies in their image formats such as CloneCD (http://www.slysoft.com) or Alcohol 120% (http://www.alcohol-soft.com)
According to the MakeABackup Games Database (http://www.makeabackup.com) Gothic 2 is protected with SecuROM 4.83.52.0004. From version 4.8 upwards SecuROM have used a method which the track lengths are not the same sizes, meaning there are some sections thicker than others. Since CD-R's have a pre-determined track density, it would be very hard if not impossible to write these thicker sectors back onto CD. However, there are some methods used to trick the SecuROM guard module into thinking it is an original.
Here are 2 noted ways.
1)Use the CloneCD+TwinPeaks+BWA Method.
2)Use the Alcohol+DPM and use that as Emulation with the RMPS option enabled.
You should be able to find these guides in the Guides and Tutorials section above.