This is not true. Which version of GitX are you using? There are a number of different forks, and the one I get from “brew cask install rowanj-gitx” works fine on Catalina. It is from 2014, but it’s code-signed and it’s 64-bit.

If you are using the older fork from http://gitx.frim.nl/ then it’s a 32-bit binary from 2009. That version won’t work on current macOS versions, but it will run on my Power Mac.

Here's the fork which I use:

https://rowanj.github.io/gitx/ (`brew cask install rowanj-gitx`)

The upstream also has some signs of life but I don't believe it has a stable release yet:

https://github.com/gitx/gitx