How do I reduce gif size when using Gif Brewry?
I’d say there are basically three things that impact the file size of a GIF. 


The number of colors in the GIF — The more colors there are in a GIF, the higher the file size. 
The dimensions of the GIF — Tumblr requires the width to be less than 500px, so at least start there. I think around 450px is a great size. However, don’t forget you can use the crop feature to just highlight the action.
The number of frames — The number of frames you can use is highly dependent on the above two points. 


The three items above mirror the advice that the vendor gives and you can use the GifBrewery preferences to help reduce the colors by processing it lightly or using dithering algorithms: