The suppliers of cast iron welding electrodes lump CGI in with ductile iron.
I think any weld repair, short of the whole part processing described in D Sen's link, risks some cracking in the HAZ, and the CGI reverting locally back to gray iron too.
Is this joining CGI components to create a structural weldment, repair of a structural part, restoration of a crankshaft bearing journal, sealing a leak, or a cosmetic repair?