There is no simple way to control GDOP (or any type of DOP such as HDOP, VDOP, PDOP or TDOP; see SOL10131 for a definition of "DOP"). DOP is a function of SV geometry as seen from your antenna, so to control the DOP you really must control (edit) the complete GNSS constellation (see Footnote 1).
An understanding of the DOP type you are attempting to control, coupled with a trial and error approach, is suggested here. Let us assume we want to maintain a GDOP value >1.7 for 3 hours.
FOOTNOTES