For more details on which models to use and which to avoide, see here:
[https://github.com/keirf/flashfloppy/wiki/Gotek-ModelsGotek models on Flashfloppy wiki]
To connect the gotek to the CPC, you can directly connect it to the DDI-1 cable connector (you would need to cut off the noses on the bottom of the cable). It can work both as A drive (no 3" drive required) or as drive B. It can even work as a 3.5" or 5.25" drive for example with parados or VDOS. Please be aware that you cannot use an unmodified DDI-1 together with the Gotek without a normal 3" FD-1 drive. That is because the FD-1 delivers the 5V power to the DDI-1 and without it, it simply does not get any power to run.