According to doc, you should be able to display the percent progress on the right of the progress bars.
This will not work because of two typos in the code. In one case, “PercentCompleted” becomes “PercentComplete”, in another case, it becomes “percentCompleted”.
Correcting these typos in dhtmlxgantt.js solves the issue. I write this so that it can be fixed in 1.4. I attach a patched version.