iMac Late 2009 Target Display Mode to GTX 1080
Target Disk mode is booting with a T and the OS not running. You will see a FireWire / USB / Thunderbolt icon in this target mode on supported iMac.

Target Display mode has no icons and needs the Mac OS to be running and is entered with either Command - F2 or Command - Fn - F2 keys.


https://support.apple.com/en-us/HT204592


The link above has all the steps to qualify / check your connections and requirements.