Bash Script su exiting shell

Question

Im trying to write a bash script that executes several commands as different users to automate an install process.

At one point I have

su gpadmin
gpperfmon_install --enable --password password --port 5432
y  
gpstar
y 

The su gpadmin is run as root, and once it is run the script stops until I exit gpadmin and return as root.

How would one switch to a new user without halting the script?

Sorry for my ignorance, I attempted to google but found nothing that worked. Any help would be greatly appreciated.