You should create an ContentNote or ContentVersion record in your test class and that would automatically create a ContentVersion record, hence, firing your
Sample code to create a ContentNote record is
// Insert a ContentNote record
ContentNote cn = new ContentNote();
string body = 'Hello, World!';
cn = (ContentNote) TestFactory.createSObject(new ContentNote(Title='test note1',Content=Blob.valueOf(body.escapeHTML4())));
System.assert([Check the results you are expecting from your GPF_Parse CSV class here]);
TestFactory is a test data generation class and you can find tons of examples on the internet on that.