How to Show Percent Complete on a Gantt Chart
Fill each bar to match how far the work has actually got, so a status view shows progress instead of just dates.
Show how far the work has actually got
Dates tell you when something is scheduled. They do not tell you whether it is nearly done or has not started. Setting progress on each task fills the bar to match, so one glance separates finished work from work still in flight.
Set a task's progress
Select a task and set its progress anywhere from 0 to 100. The bar fills proportionally as soon as you do. The fill renders whenever progress is set, whether or not you show the number.
Turn on the progress label
The fill alone shows the rough shape. Turning on the progress label prints the exact percentage as text, and you can place it in any of the usual label positions around the bar.
For the second half of that picture, see tracking schedule slip with a baseline.