Check this book out; it seems to be talking about that particular grade of steel used for reinforcement.
The choice of main bonded reinforcement is based on its high tensile strength and interaction between it and concrete, resulting in higher bond length and an improved degree of crack control. A considerable experimental work has been carried out by research on steel bars such as GK 60 deformed bars in close cooperation with the manufacturers. Experience of design companies with this material in the past suggests to use GK + 60 1 1/4" dia. (31 mm) bar as main bonded reinforcement steel. It develops a stress of 60,000 psi at 0.2% strain and has an ultimate strength of about 100,000 psi (735 MN/m2) at minimum guaranteed elongation of 14%. The low carbon content (comparable to mild steel) of GK 60 reinforcing bar simplifies welding in local areas and can expect to carry the full tensile strength of the bar. Bond tests carried out on G.K 60 according to BS8110 of EC-2 indicate that where an end slip of 0.001" occurs its bond strength exceeds a plain bar by more than twice the 40% suggested by B8110 of EC-2 and with initial end slip, the anchorage value increases continuously up to the maximum value. This suggests that by using this type of reinforcement, no hooks are required in the vessel concrete. All main bonded reinforcement shown on vessel drawings cannot be less than 1 1/4" dia. (31 mm) high tensile GK + 60 bar. Minimum requirement of bonded steel in different areas of the vessel is, therefore, based on the properties of GK + 60 bar or similar. For other types approved by authorities, special tests shall be carried out.
And this link (the paper is free to download) has a little more info.
And this forum post too:
In the UK, GK-60 used to stand for Guest Keen & Nettlefolds (South Wales Limited) bars manufactured from grade 60 steel with the minimum guaranteed yield stress of 60,000psi (described also by the manufacturer as Hot Rolled High Yield Deformed bars). The tensile strength is at least 15% above the measured yield load. The guaranteed minimum elongation is 12%.
It is also claimed by the manufacturer that it has enhanced (by 40%) bond stresses. For the designs according to the CP114 (UK permissible stresses based code) the anchorage lengths should be between 40 and 50 diameters, depending on the bar diameters, stresses in the bars and the concrete mixture.
Above is based on the GK&N (South Wales) Ltd. brochure from 1965. Hope this helps.
Google "GK60 bar", there are lots of details, especially in the books section of the searches.