Hello : )
I assume you will open the testimonial graphic as an image (TIFF, GIF, PNG, etc) on Gimp! It will open as it's own layer (that is what I suggest, make it the base layer [background layer]).
Use the rectangle select tool (it should be the first tool on the tool window [on my GIMP it is XD]) and draw a rectangle (or any shape you want) around the text you'd like to get rid of. Then under file or edit, select "clear" (short cut ctrl+K). And it should remove the text. o.o
But oh! There might be a white space to fill, huh? ._. That's a problem, isn't it? Dx
Perhaps you could just remake a testimonial graphic to look like the one you want. : ) Or, if you're not that skilled in GIMP or don't know how, ask someone you make one for you, customized! I would do it for you. o.o
Anyways, it really just depends on if the background color of the actual testimonial graphic is white, or blue, or any other color OR has some sort of gradient/pattern applied to it. If it's not too complex of a pattern/gradient, you could use the copy tool (the STAMP) and try to fill in the white space created from removing the text.
I hope this doesn't sound too complex x..x I really don't know your skill level in GIMP, so yeah. :X