There's indeed already a GFF parser in Geneious.
You can simply call
The importer will automatically detect that the file is in GFF format, and apply the results accordingly, so you don't have to transform it to a TextDocument.
It will take the currently selected folder as the destination, if you want to put the results into a subfolder you will have to set that folder as the selected service before calling the importer.
Also, if the resulting annotations are supposed to be applied onto an already existing sequence, that sequence should be present in that folder, or even better, should be the selected document.
The importer will automatically detect the most suitable sequence to apply the annotations to, based on the name in the GFF file and what is currently selected or available in the selected folder, if it doesn't find a sequence with matching name to what's specified in the GFF file it will show a dialog to ask for user confirmation.
I hope that helps, please let us know if you have further questions.