apart from making sure whatever i'm screencapping for gifs is HD (i tend to go for 1080p usually, sometimes 720p), i mostly play around with curves and levels for darker gifs.

going to use this as my example

haven't added any adjustment layers to it, but usually i start off with curves.

i DON'T use the highlighted one (i tend to use that for any scenes set in daylight, lighter backgrounds but not for darker ones like this because it just ends up making it look too dark). here's what it looks like after this layer is added.

i do, however, use the middle dropper for colour adjustments for getting rid of blues or yellows

and this is what it looks like now. added another curves layer and dragged the top right corner point to the left to increase more brightness (in addition to the brightness/contrast layer)

to make up for not using the top dropper in either of my curves layers, i then add a levels layer with these settings and no dropper selected (can vary depending on how you want the gif to look)

and here's what it looks like after that