Just moved into a new property which is fitted with a shower tray. Previous property had a pump fitted to the shower tray drain.

I removed the cover off the the drain outlet of the shower tray and it is disgusting, full of gunge and dirty stagnant water.

Should shower trays be fitted with a drain pump? to avoid all of this gunge building up?

What thescruf said.

The vast majority of houses don't need a drain pump. None of the 6 houses I've owned needed one. They are usually only needed where the drain pipe can't slope down and away. For example in a basement flat.

There should be a "trap" or "U bend" under the shower tray. This is designed to stay full of water to stop sewer smells coming back into the bathroom. It's not unusual for the trap to get pretty full of gunk. Clean it out best you can, flush with water and see how you get on.