How to manipulate the text to have half gradient or half pattern?

[rich2005]

If it is just a one-off / infrequent effect. Another way:

Layer -> Duplicate layer
Layer -> Transparency -> Alpha to selection
Fill with pattern / colour
Layer -> Mask -> Add layer mask (white)
Black FG paint out the layer mask as required:

50 second example https://i.imgur.com/u1XL0so.mp4