With the OSSC, the refresh rate you feed in is what comes out, so you feed in PAL you get PAL out of the DVI/HDMI. Therefore, your TV/display must support PAL for this to work.
I wouldn’t bother with a stepup/down if you can just replace the power brick/wall wart. I’ve never owned a GX4000 so I don’t know what it uses but if you’ve done your research right that PSU should work. Beware of very cheap PSUs as they can add picture noise.
The SCART cable you linked to is just a SCART to SCART. Would work for going from GScart to OSSC of course.