How can I update several installations at the same time via ssh?
For example one grav-installation in folder A, a second grav-installation in folder B.
"bin/gpm update" is only working within a folder.

See this post for a very efficient approach using Deployer.

Got help and found a solution. :-)
Upload a Script (for example update_grav.sh) with the following content:

----```

!/bin/bash

logfile="/path/to/log/upgrav$(date +%Y%m%d)1.log"

j=0
Setups[j++]="/path/to/installation/grav_installation1"
Setups[j++]="/path/to/installation/grav_installation2"
Setups[j++]="/path/to/installation/grav_installation3"

Add more grav setups as above

echo "### $(date +%Y%m%d_%T) starting update of grav installations " 2>&1 | tee -a ${logfile}

for i in "${Setups[@]}"
do
echo "### $(date +%Y%m%d_%T) update directory $i" | tee -a ${logfile} 2>&1
cd "$i"
bin/gpm selfupgrade -y 2>&1 | tee -a ${logfile}
bin/gpm update -y 2>&1 | tee -a ${logfile}
done

echo "### $(date +%Y%m%d_%T) finished update of grav installations " 2>&1 | tee -a ${logfile}

----```

Than, start your script with "sh update_grav.sh" and relax.