For running J!5.1 you must https://fabrikar.com/forums/index.php?wiki/update-from-github/ or include the new file manually https://fabrikar.com/forums/index.php?threads/joomla-5-1-and-fabrik-cannot-find-files-error.54473/post-285151 See also Announcements
Please see our announcement here.
Thanks @juuser, I'll give it a try.You can still use FusionCharts latest Gantt charts "manually":
https://www.fusioncharts.com/charts/gantt-charts/simple-gantt-chart?framework=php
You just need to add the chart generating code e.g. to custom module and you can embed it to list or form with Regular Labs Modules Anywhere. It's a bit more complicated as you need to build the chart data with your own query, but nothing too extreme.