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GIF will allow you single-color transparency, while PNG has full alpha channel support.
This is for text along a circle in the bottom of the circle. For the text on top of the circle, omit steps 3-5.
See also Text Along Path - GIMP 2.8 Tutorial For Beginners
Resynthesizer allows you to remove watermarks and similar from an image. Download it from the GIMP Plugin Registry or my wiki. Move all files to GIMPPortable\Data\.gimp\plug-ins\.
For an example, see GIMP Watermark removal