system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 What are gamma values in pictures and why does it affect png (aka make it worse than jpeg?)
gpt
This comes from the days when computers were much slower and monitors were huge and heavy boxes with thick glass on the front of the screen - the CRT, or cathode ray tube, uses intense electromagnets to shoot tiny bits of stuff at a screen that glows when hit by this stuff. That's how it makes colors and pictures, by changing how the stuff is fired at the screen and by aiming to make flat lines to cover the whole screen down from the top, 30 or 60 times every second. 

When this was going on, in the earlier days of computing, we hadn't figured out how to make colors exactly the same from Mac os and Windows and other computer operating systems. When we tried the normal way they looked different, and that did not work for us. 

So we tried to use this gamma value to preserve the color we would see on the screen from one device to another to make them match. But something went wrong when not everyone agreed on how or even if they wanted to use it at all. So all sorts of things happened, some devices used it right and others did not. It was a mess!

The gamma value specifically tells the computer how much of the color to show, from none to all of it or possibly even more. That's because the gamma was used to say 'this device is already showing more green and red and blue than the one that made the image, so turn down the colors of this image so it looks right on this device' or 'this device naturally shows less green and red and blue than the one that made the image, so turn them all up to match'.

Over time we realized how silly this was and fixed the problem in a better way so everything matched all the time outside of gamma images. Now the gamma value is a leftover of what was tried before, and most browsers don't use it or have a default set up to work in most cases.

Better or worse depends on what you're doing. PNG is designed to be good on websites, so it can get smaller than a jpg or gif with the same picture. But not all browsers support it the same as jpg or gif formats, there are still old browsers out there still using the gamma values or trying to. Because of that it is a matter of knowing who will be using your site and images - if you need to use the old stuff jpg may be a better bet. If you have all new stuff png may work better. When you make new png files, the gamma can be left out to make it like a more efficient jpg format. 

Great question!