In some rare cases the Gantt view can show certain records for one user but not another.
This is the case even though they have the same privileges and sharing permissions. This can be due to the users having different time zones in their user record.
More specifically, records in question will most likely be those on the border of the time frame. e.g. at the end of the 6 month view, 1 year view, etc…. To test if this is the case, try verifying and changing the user’s time zone to see if that changes what records appear for one user and not the other.