I want to find out which developer had commited a particular block of code in a particular file.
I don’t think there’s a feature for this in Gitlab but may be there is.
Anyway this question is more related to Git itself than Gitlab and the answer is:
git log -S'the line from your file' -- path/to/your/file.txt
Only since git 1.8.4
More details here for example: http://stackoverflow.com/questions/8435343/retrieve-the-commit-log-for-a-specific-line-in-a-file