It has been a long time since I looked closely at a Raleigh GP, to double check that 700c is going to work, make sure you can lower the brake pads 4 to 5mm, that is the difference in radius between a 27" rim and a 700c rim.
27" quality high pressure tires seem to be have already gone out of style, so if 700c will work, go forth.

Nottingham GP's would often have Raleigh threading, 26 threads per inch, not standard outside of the Raleigh MFG. kindom...Except Gazelle built frames could easily have English, 1.370" x 24 tpi, LH threading on fixed cup side of course. so test, compare upon removal of the adjustable cup to a known English threaded unit. Even if it has the "odd ball" threading all is not lost, a tapered spindle with the correct spacing and other dimensions could be used, we did it 30 years ago but finding a cooperative bike shop (or knowledgable one today) could be a problem.

be seeing you