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Inputs: SCART adapter, optional sync stripper
If you want to build an adapter, here is some good inspiration: https://shmups.system11.org/viewtopic.php?f=6&t=35423
The ArcadeForge Sync Strike is a solution that will work, if you don't want to build an adapter yourself.
http://arcadeforge.net/Scaler-and-Strike-Devices/Sync-Strike::15.html?language=en
Every GBS board comes with ~500 kOhm termination on the sync input.
This termination is meant for VGA and most arcade boards. It is unfit for most other sources however.
If you want to use the RGBS input for regular TV level sources (game consoles), the GBS requires one additional 100 Ohm resistor to ground on the sync input.
Together with the factory resistor of ~500 kOhm, this will bring the total termination close to TV levels of 75 Ohm. Note that by default the GBS board only accepts CSync; For converting RGBS video signals that use Sync-on-Luma, Sync-on-Composite and others, see below.
Also note that total resistance is closer to 82 Ohm with this extra resistor. This is intentional, as it leaves more headroom.
Better compatibility can be achieved with a sync stripper, such as the LM1881, which can extract CSync from various RGBS signals.
Please build your circuit so that it has a 75 Ohm to ground (termination) resistor on the LM1881 video input, as well as a 470 Ohm series (attenuation) resistor on the sync output.
The 75 Ohm input resistor is the required termination for the source.
The 470 Ohm series resistor on the CSync output of the LM1881 lowers the voltage to safe levels for the GBS.
Your extracted CSync can now be connected to the "S" (sync input) pin of the GBS.
User viletim shows a good LM1881 circuit here:
https://shmups.system11.org/viewtopic.php?f=6&t=55948&p=1153713#p1153713
https://shmups.system11.org/viewtopic.php?p=1153077#p1153077
Note that his circuit omits the Rset resistor and capacitor. This works for CSync generation, but the VSync output will not work. This is not a problem for this application, as we are only interested in the CSync signal.
Also note that the LM1881 requires 5V to operate, but that the GBS only makes 3.3V easily available.
5V for the LM1881 needs to be sourced elsewhere, from the GBS power input, for example.
I successfully tested the sync stripper at just 3.3V, with the output 470 Ohm resistor removed.
This is not recommended though, and the LM1881 will have a hard time with some signals.