J’ai enregistrer un un projet dans mon ordinateur mais une fois le logiciel fermer impossible de le retrouver.
We do not speak French, sorry. Here is your message translated with Google Translate:
I saved a project on my computer but once the software was closed, it was impossible to find it.
Please consider writing in English for better understanding.
I am not sure that I clearly understand how can I help, but anyway, chances are that you’re on macOS and you didn’t try to drag GanttProject into Applications, which is the usual way of installing apps on macOS.
See this reply: Program gone from desktop - #4 by DmitryBarashev