PV: Can I use picture messaging on my iPhone with the GSM network?

Yes! For the GSM network, just make sure your GSM iPhone is running iOS operating system 13.3 or better. The oldest devices that can accept this iOS update are the iPhone 6s and the iPhone SE. To update your phone's operating system, turn your phone on and then go to Settings. From there, tap on General, then Software Update. Tap Download and Install if you see an available update. 

Click here for more information on updating your phone's operating system.