You can have functions in the PS1 variable, just make sure to single quote it or use escape for special chars:

gitPS1(){
    gitps1=$(git branch 2>/dev/null | grep '*')
    gitps1="${gitps1:+ (${gitps1/#\\* /})}"
    echo "$gitps1"
}

PS1='\\u@\\h:\\w$(gitPS1)$ '

It will give you a prompt like this:

user@Host:/path (master)$

Notes:

• Make the changes in ~/.bashrc or /etc/bashrc or ~/.bash_profile or ~./profile file (depending on the OS) and save it.
• Run source ~/.bashrc (distro specific) after saving the file.