If you want to set environment variables permanently in Git-Bash, you have two options:
Set a regular Windows environment variable. Git-bash gets all existing Windows environment variables at startupp.
Set up env variables in .bash_profile file.
.bash_profile is by default located in a user home folder, like C:\users\userName\git-home\.bash_profile. You can change the path to the bash home folder by setting HOME Windows environment variable.
.bash_profile file uses the regular Bash syntax and commands
# Export a variable in .bash_profile
export DIR=c:\dir
# Nix path style works too
export DIR=/c/dir
# And don't forget to add quotes if a variable contains whitespaces
export ANOTHER_DIR="c:\some dir"
Read more information about Bash configurations files.