I don't have any specific info on entrance to SGS at any time other than the normal 11+ time.
However, the website for SGS takes you to this link http://www.salegrammar.co.uk/files/user ... Policy.pdf
If you scroll down it you come to the section dealing with admissions to Y8-11 and you will see that for Y8 the student will be tested in VR, NVR and Maths.
Have you tried ringing the school and asking Admissions for info re timings, difficulty? It is possible that the tests will be the same as the 11+ questions but the child is expected to answer the questions in less time - that's how other LEAs like Bucks manage the process, but I haven't come across this in Trafford before so am as much in the dark as you.
If you do phone and find out please come back and let us know.