Accidentally uninstalled ECG app from Apple watch series 5

Open the Apple Watch app on your iPhone:

• In the app, go to My Watch (the default tab at the bottom left of your screen).
• Scroll down to see the list of apps that are installed on your iPhone (if you also deleted the app from your iPhone, first reinstall it via the App Store app)
• Then tap on the name of the app that you want to put back on your watch and enable "Show App on Apple Watch".

There is the ECG in the App Store. It's closer to the bottom when scrolling. It says ECG then underneath that "Utilities". On your Watch tap on App Store ant using the microphone say "ECG App"...then scroll down to it.

Glad you have it back again.