The GFF file format is a standard format which was originally designed for listing annotation features, but which has also been widely used for large sets of mapped data.
Since the fields in a GFF file are defined you don't need to tell SeqMonk anything about the file to allow it to be imported. You simply select the files you wish to import. You will see the common import options for single end data only since GFF does not support paried end reads.
Within the GFF format the first field is the name of the reference sequence. To import a file into SeqMonk this name must be the chromosome name (eg 1 or chr1).