Buenos días, Es posible dar formato de autosuma a una columna agregada personalizada, Ya que agregue una pero las filas padre que no están indexadas no muestran algún numero.
Anexo una imagen 
We do not speak Spanish, sorry. Here is your message translated with Google Translate:
Hello, It is possible to autosum a custom aggregate column, since I added one but the parent rows that are not indexed do not show any number.
Please consider writing in English for better understanding.
I am not sure that I understand what is “custom aggregate column” (we have no such columns) but anyway, custom column values are opaque to GanttProject and it won’t do anything with them.
Fila padre = Tarea resumen, que siempre contiene tareas.
So, seems that there is no such “custom aggregate” function for summary tasks.