For a display with HDMI input, an RGB SCART to HDMI converter is a good choice. A SCART RGB cable can be purchased from many european sellers.
For an example start around 3:15 at
https://www.youtube.com/watch?v=sryNIHhW23w If you're looking for VGA, I'm not sure if there's a good answer.
The GBS-8200 is the low cost VGA solution and is designed for CGA (digital 15kHz), not analog used by the Amiga, Atari ST and Apple IIgs. Not being analog friendly is such a problem that Microbee created a front end board; see
http://www.microbeetechnology.com.au/vga-video-converter.htm