Clearing canvas texture from photo of artwork

[rich2005]

(03-04-2019, 01:49 PM)dmaarie Wrote: .... I would like to keep the grey background in tact.

Once you have it adjusted.

Layer -> Transparency -> Add Alpha Channel then Colours -> Color to Alpha

New layer, move under the sketch, fill with grey. something like this. https://i.imgur.com/2M8FXSm.jpg
or,
Fill with a canvas type pattern, depends what is required. 

Edit: This with just a hint of a texture