EN-GJL-250 is a designation for cast iron. This particular grade is equivalent to ASTM A 48 Class 40B.
Ck45 is an old DIN designation for the grade now called C45 (DIN EN 10083-2), with "+N" indicating that the material is to be supplied in the Normalized condition. In the US, this grade is called SAE 1045 according to SAE J403. ASTM standards are for product specification (bar, sheet, etc.), with ASTM A 29 and ASTM A 108 being just two of many ASTM standards that reference this grade.
42CrMo4 is another DIN EN grade (10083-1) that is equivalent in the US to SAE 4142 according to SAE J404. Again, this grade appears in various ASTM product standards. The "+QT" means that the steel is to be supplied in the Quenched & Tempered condition.
18CrNiMo7-6 is a case-hardening steel that appears in DIN EN 10084. There is no direct US equivalent for this type of steel, due to the combination of high Cr (1.50-1.80) and high Ni (1.40-1.70) with 0.25-0.35 Mo. Typical case-hardening steels used in the US are SAE grades 4320, 4817, 4820, 8620, and 9310. Use the following link for information on the chemical compositions of these grades: