A GFF file does not contain any sequence information, it only contains a list of annotations. You can read more about the format at http://www.sanger.ac.uk/Software/formats/GFF.
You can also use GTF files (Gene Transfer Format). This format is used by among others the UCSC Genome Browser. If you download an annotation track (e.g. SNPs or repeats) you can get this as a GTF file.
Note! GTF-files downloaded from the UCSC genome browser are not compatible with the RNA-seq analysis of CLC Genomics Workbench because the gene and transcript annotations cannot be matched. See examples and guides, and read more about the plugin in the user manual.