sometimes ive found the problem is worse when your constantly looking at it,if say i go away and have a rethink and come back to it later refreshed i sometimes find i can solve it a bit better with a better frame of mind.
EDIT: ive just had a thought.are you booting up workbench from harddrive?
if you are, im wondering why you not setting it to ntsc there.but ive got a guess
from harddrive, open your boot partition click on storage open monitors and look for ntsc(its a monitor dos driver)
then open devs and open monitors(im assumming here that theres only the pal one)
then drag and drop the ntsc dos driver into the monitor drawer in devs and reboot.
then when it boots click on prefs and click on screenmode and pick a ntsc mode and click use and save,then when you reboot again it should be in ntsc.
its also worth noting that the gbs doesent like the overscan prefs on mine for some reason but i'll get into that later after this.
Yes, I have the CF card with WB on it, that boots directly on it.
Tried as you said, and it works; I can basically boot, get 5 seconds of green weirdness and then WB comes up.
Altho that's the only thing that I can use

If I run a demo or a game, or even a program like personal paint, which changes the mode to the standard mode of the machine, I loose the signal. I don't get the green weird screen, nor the card show me the "no signal" message, but I get a black screen.
Basically it is almost unusable in this way T_T There are very little programs that does not open a screen in their own resolution, so I am very limited to what I can do with the Amiga, if I use the GBS.
Checked also on the manufacturer site, and their technical service has no clue of what is going on; they say to try it with a JAmma card, because that's what should work for sure...thank you very much

I've asked if there is a way to change the firmware and they said that it can be done only in the factory. They ship to various distributors, cards with slightly different firmware on it; which would explain why in some cases it works just fine.
At this point it is a lost cause to continue to spend time trying to make this thing to work....got today the SCART/HDMI adapter, and will give it a try as soon as I finish the scart cable. Tried with a composite to scart adapter, and the picture was really nice and smooth, but there was a heavy blurring on the text in WB, which makes me hope that it is just a problem caused by the fact that I was not coming out from the miggy via rgb, but using composite
Thanks again for the help! will post some pictures as soon as I am done with the RGB-SCART cable!