3 possibilities

a) If your Gefen product (which specific one ?) has a menu it should allow you to adjust image size and position

b) Click on the button labeled "Monitor" and, depending on driver version (AFAICR) you will have options to adjust screen size and centering . See [1] for example screenshots relating to TV out, VGA monitor options will be similar (AFAICR), but I can find any screenshots .

c) Try PowerStrip

[1]
https://www.anandtech.com/show/389/6

as is the case with Old 386 PC with CRT Tv those types of converters need to sample incoming video data and then format it accordingly it, the easiest way is to hardcode some common timings (resolution/refresh rate) and simply ignore the problem of auto detection
tldr: shit scaler
easiest solution is buying cheap big LCD with VGA input just for retro activities

rasz_pl wrote on 2022-11-05, 12:27:

as is the case with Old 386 PC with CRT Tv those types of converters need to sample incoming video data and then format it accordingly it, the easiest way is to hardcode some common timings (resolution/refresh rate) and simply ignore the problem of auto detection
tldr: shit scaler
easiest solution is buying cheap big LCD with VGA input just for retro activities

I've got a working LCD that I use for actual display. This is to feed a capture card.

hewh googling this gefan thing one of the first results is Amazon and 2012 review https://www.amazon.com/Gefen-EXT-VGA-2-DVISP- … iews/B000VBDMSU

". I only needed basic 640x480 VGA resolution to work-- this is as simple as it gets. However the Gefen device would not convert it properly. It would cut off about 10% of the right-most portion of the video signal. I tried all the settings in the menu to no avail. The only setting that has any affect was the "clock" setting-- with the clock set to "100" I lose about 20% of the right part of the video, with the clock set to "0" I lose about 5% of the right part of the video. Too bad the adjustment doesn't go further because then it would probably work."

so yeah, shit scaler. You could try opening it up and looking what chip was used, maybe someone had a project reprogramming it, or configuring it externally with arduino or something