Gantt report is very heavy to produce.
And convertion to PDF is also very heavy.
That's why gantt planning may come to either "Memory limit" issue or "Timeout" issue.
Only solution if you want to Export to PDF is to increase memory_limit and max_execution_time in php.ini.
But this won't whange that it may be very long to produce.
Moreover, this report does not include all information in the gantt chart, such as dependencies.

That's why we propose a PDF export on the Planning view, that uses completely different technologies.
It is quick, lightweight, and has no impact on server as it is produced on browser side.