Morning George,
Choose file-export, change the file type to gif.
Give it a name and press export. On the "convert to bitmap" window check the filesize is as you want it, and press ok.
In the "convert to paletted" window, choose "optimized" in the palette drop down menu, click ok
On the "gif export/transparency" window put a tick in "image colour" click on the eyedropper tool to select it. On the left hand side of the window (original image) click on the colour that is to become transparent, press the preview button to check it in the right hand side of the window. When happy press ok.
Best wishes
Andy