"The ".grd" file suffix is recognized by Chimera as a UHBD electrostatic potential binary file. But your file is a text file as can be seen in any text editor. A look at the rdock documentation mentions it uses InsightII grid format

http://www.ysbl.york.ac.uk/rDock/documents/rDock_reference_guide.html

and that format is described here

http://hincklab.uthscsa.edu/html/soft_packs/msi_docs/insight980/insight/Utilities.fcs.html

and appears to match your file header which looks like

RBT FFT GRID
(1F15.10)

20.500 17.500 18.500 90.000 90.000 90.000
41 35 37
1 -33 8 -35 0 -5 32
0.0000000000
0.0000000000

...

Chimera does not read Insight II grid file format. It is not much different from some other text grid file formats like DX format, or CNS format, or XPLOR format, which Chimera reads. You might be able to convert to one of those formats in a text editor. It would also be possible to write your own Chimera file reader in Python if it was worth your effort."

I've attached a Python script insightgrid.py that will open your grid in Chimera using Chimera command

runscript ~/Desktop/insightgrid.py ~/Desktop/2C6U_rdock_mdmix_R4_cav1.grd

The grid does not align with the standard 2C6U PDB file as shown in the attached image but maybe you used a shifted or rotated version of that PDB.

Tom