understanding gamma settings?

I got a 2nd hand sony 21" crt today and im having problems trying to calibrate it
i am using adobe gamma and i dont have spyder or any fancy kit yet but im having a nightmare trying to understand the gamma settings and how it works?
it says i have to adjust the slider until the grey box merges into the lined background but it just is not happening for me. The monitor is set to srgb with a temp of 6500k but the color looks off to me. It seems to have a slight green tint also???

any advice(apart from getting a calib device) on how to sort this out or how to understand Gamma settings??

Green tint - could be the guns in the CRT circuits are not set up correctly or there could be some other problem which is why the original owner has sold it.

Can you display a known white from a calibration chart or from a weblink? What is the colour tint for that - if all three guns are properly adjusted the RGB should combine to give a white without any tint - bearing in mind that a colour temp of 6500k (which is correct) will have a slightly warm look to it - but definitely not green!

If it is still a green tint then you might need to have the CRT electronically adjusted before attempting monitor calibration. A TV/Monitor service engineer (if you happen to know one) should have the appropriate signal injection equipment and the know-how to set it up.

It would be my first stop before looking at calibrating.