I've seen some great vector illustrations using some great looking textured gradients but the only online tutorials I have found use Adobe Illustrator. So I set out finding a way without splashing out on the expensive monthly subscriptions.
Start by creating the vector shape you wish to use.
Switch over to "Pixel Persona"
Create a pixel layer then drag it into your shape.
Switch over to Export Persona.
Make sure the correct layer is selected, then choose the best quality jpeg and tick "x3". You might be warned about the resolution but go ahead anyway as the larger size will help in future steps.
Open your exported texture into Image Vectorizer app, then adjust the saturation, contrast, brightness and gamma until you have the gradient you desire. Hit "Vectorize" when you are happy and then copy your image to the clipboard.
Paste your vector texture into a new document. Then select small groups of layers, "Add" them together to make one complete shape/layer which makes the final illustration less complicated. I found trying to combine too many layers caused Affinity Designer to crash.
Copy your complete vector shape then paste into your illustrated document to resize and colour as you wish.
If needed you could then duplicate your vector and re-size, flip or stretch to create the effect you need.