First of all. It seems that this question of yours is a followup question on your other topic 'Hello...My 3dmap is very dark'. It's considered (n)etiquette on forums to not make new topics if your new question still belongs in the first. Only create a new topic if your question is about something different. No need to clutter the forum, is there? There's a grey 'post reply' button at the end of every thread.
Now for your question...
Gamma is a curve-factor in the color-transformation function done either by software or hardware. You may have this function in your videocard/desktop properties or your monitor may have this functionality (just like contrast/brightness buttons). Your family members that use your computer also wil appreciate this very much.
DW has it's own built-in functionality for this with a line in a config file. You have to search the forum on ways how to do this exactly. Never had to do this myself. SC does NOT have this built-in function!
Gamma value equal to 1 means dark to light is linear
gamma between 0 and 1: dark to light becomes an 'increasing steepness' curve (dark becomes much darker than light becomes darker relatively speaking)
gamma bigger than 1: dark to light becomes 'decreasing steepness' (dark becomes much lighter than light becomes lighter relatively speaking)