Customized Adapter with Ex Brass Gland

If the equipment it is to screwed into is Ex d, Ex e or Ex p, then no, it may not be done within the hazardous area. Nor is it permitted to open-out the hole in the equipment to fit the gland that is currently to hand without doing approval testing on the modified equipment if that equipment is in the hazardous area.

Choose instead the correctly-sized, rated, gland for the enclosure into which it is to be screwed.

Not permitted as an adhoc modification but technically possible if accepted practices and proper materials are used.

Find out if the enclosure manufacturer has a tech memo on this and follow the instructions.

I doubt you are the first to encounter this.

Depending on the application environment, both technical and litigious, you can proceed with informed confidence or err on the side of caution.

PWSlack is correct.

I wish to add that when tan item is certified, it means 'manufactured to specification for use in a described Ex area/s.

Either your cable spec is incorrect, or the gland is faulty in manufacture. Suggest you compare with another fitting, or measure accurately and refer to your supplier to confirm dimensions against the manufacturing data sheet.

Ensure you are not confusing the certification eg Ex(d) with Ex(e). There may be a difference in orifice size...