There are several textures in the ADE-GP texture folders that illustrate adding text to GPs. Look in the Textures_Dpy folder. Note that if you create your own textures you'll need to also make entries in the Textures_Def.txt, and perhaps also the Lines_def.txt files. The process is well documented in the ADE-GP USer Manual.
Yes, I am aware of the textures in Texture_Dpy folder and have used these to create custom poly lines. But I can't understand how to use the text. For example, if I select gp_Signs_Yellow.bmp then how do I choose the text on it? The manual, unfortunately, does not explain this.
You select the text you want in exactly the same way as you select a portion of any other texture; in the GP Editor, move the vertices of the poly so they enclose the portion of gp_signs_Yellow you wish to use. The text is not special, its an integral part of the texture.
To elaborate - create the polygon the size of the character you want. Then the editor opens an choose the texture. Then move the vertices to the same shape as the polygon, surrounding the character you want. If you want to create words (i.e. multiple characters), you create individual polygons for each character.
If you don't want individual polygons for each character then you need to create a custom texture with the words you want.
The only thing I would add to Tom's useful instructions is that, if you want to have the letters appearing on a block of a different colour, then you have to create a GP Poly and make sure that it is on a 'lower' layer than the letter GP Polys.
Very good point raymk, many markings are on a block of red, etc. The layer value will determine which GP displays on top of which. Make sure the letter polygons have a higher layer number than the single color block polygon and the letters will appear on top of the color block. If you look at my second image above you can find the layer number at the top left, set to the default value of 24. Set the color block polygon layer to a lower number here (or set all letter polygons to a higher number). Some combinations will work better than others, and many use layer numbers 4 layers apart for the older style (FS9 and FSX) GPs (20, 24, 28, etc.).