If you are looking for Xtns menu of toolbox, don't, as, starting with Gimp version 2.6 the Gimp developers have removed the toolbox menu bar and merged it with the image window menubar. To make a logo, go to the image window menu bar. Select File -> Create -> Logos. Under Logos, you may choose one of the given styles, say Basic II. A Script-Fu dialog opens. Here, you can enter the text for logo, font size, font, background and text colors.
Once you press OK, logo is displayed. If you do not want the background, open the logo file in Gimp, go to Layers dockable menu and delete the Background layer. Save the logo image in PNG format.