ROS kinetic installin gazebo 9 rather than gazebo 7

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Can you please provide the exact list of commands you ran, the full console output and describe precisely what you are trying to do ? Several things don't seem right is the questions making it harder to answer.

When I was use the following instructions for ROS kinetic with raspberry pi, it is trying to install gazebo9

That tutorial does not result in installing Gazebo (it explains how to install ros_comm, robot or desktop, none of which contain Gazebo); so you must be diverging from the tutorial somewhere.

Running Gazebo on the RaspberryPi will produce poor performance at best, Gazebo is a heavy desktop application that is not very usable on small platforms. If you don't need Gazebo or can run it on a remote computer instead I strongly encourage you not to install/use it on your Pi.

I am using debian stretch

ROS Kinetic targets Debian Jessie and not Debian Stretch as listed in REP3, if you have the choice of the version of ROS I recommend you to use ROS Melodic that is more recent and targets Debian Stretch.

I am using debian stretch which can't install gazebo9.

The ROS repositories do provide gazebo9 packages for Debian Stretch, so Debian stretch can install Gazebo9

it is trying to install gazebo9 rather than gazebo7

Debian Stretch comes with gazebo7 by default. But ROS provides additionally a gazebo9 package for Stretch (for ROS melodic users). The most likely scenario is that you are trying to install BOTH gazebo7 and gazebo9 and this is not possible.

sudo apt remove -y gazeb* libgazebo* libignition* && sudo apt autoremove -y
sudo apt install -y gazebo7

When you run rosdep install you should tell it to exclude gazebo so that it doesnt try to install gazebo9:

rosdep install -y ... --skip-keys gazebo