1/ With your code and the PlantUML online server:
On gantt diagram, from:
skinparam classFontColor red
skinparam classFontSize 40
If you would change the font size of Gantt diagram, you must change gantt font size (not class font size), and not by skinparam but nowadays by style, as:
See also help on:
2/ About pypi plantuml 0.3.0
Does plantuml .3 support font modification?
It is very old (plantuml 0.3.0 - Released: Nov 1, 2019); perhaps it does not support style feature....
But if it calls a plantuml server, perhaps it will be good...
Go to test... and confirm...
If that can help,