How do you git fetch all repos in a folder?
People may answer git commands that does it, but looking at your intent towards GUI, here's my answer. 
I use SourceTree. It also gives the ability to add scripts as custom functions. fetch gives these options. It also has nice support for submodules. 



Here's a custom action that I use. I just picked up the condition code from stack overflow. But git would also exit if there are conflicting changes. 

cd "/Users/me/project/"
if git diff-index --quiet HEAD --; then
    # No changes
    echo "no changes"
else
    # Changes
    echo "changes"
    exit
fi

if [[ -z $(git status -s) ]]
then
    # No changes
    echo "no changes1"
else
    # Changes
    echo "changes1"
    exit
fi

git checkout master
make update
echo "made update"
git checkout feature
git rebase master
echo "rebased"
exit