do_dssp is just a wrapper that calls dssp, mkdssp, xssp, or whatever_they_call_it_now. You don’t invoke the DSSP program manually to analyze a trajectory. $DSSP needs to point to the (in your case) mkdssp binary and you should do the analysis with do_dssp.
gmx editconf -f input.gro -o output.pdb
But again, you don’t need to do this for the analysis you’re seeking to perform.