Hi Mario,

There is no way to add headers or footers to your gantter project file, wether you are in a trial or have already purchased doesn't matter. However It is possible to add in your company logo to a printing copy of Gantter. If this feature interests you follow the steps below. 

You will find this feature in you account settings in the top right corner of you page. You will click on your email address or profile photo. Manage account menu will pop up and you will be able to select the print tab. 

Sorry we do not have the feature you are looking for, let me know if you have any other questions. 

Kind Regards, Sophia