Licensed Fonts
The fonts used in the InDesign document may have some restrictions, which are usually connected to licensing. Most fonts need to have a license in order to process them for printing. When the font is missing its license, it can create issues while generating a PDF.
It is necessary to buy a license for fonts which require it in order to use/modify/process them. The license should be set for multiple users since the InDesign document passes through different users/designers.
If the font does not have any restriction on usage, then it can be used any number of times and it would not create any issue with the further process.
Also, when you upload files to Gelato please ensure that the Gelato preflight error report does not show any font-related issue. The fonts would not display properly on Gelato if they are uploaded without rectifying the issues.