This is a tutorial Michael Stephan sent me regarding editing his roadname textures. It seems to me that the concept will also work to edit the GP15 logo texture. The issue with the Mopac logo looks like it was not created from a sharp original image.
If you have DXT2BMP and RW_Tools installed you already got all you need to edit a texture and its alpha. First, follow the standard procedure to convert the TgPcDx to dds with RW_Tools. Then open the exported dds (see RW_Tools's temp folder) in DXT2BMP.
In the case of the name (or any autonumber) textures, first, go to the IMAGE menu and "send to editor" the main image. To make you life easier, you should choose your preferred paint tool first in the PREFS menu of DXT2BMP (it defaults to MSPaint). When your image has loaded in the paint program, flood-fill all of it with the desired color of your name/number font. Use a light grey if you want it to appear white in the sim. Then save the image file (overwrite yes). Return to DXT2BMP and click on "Refresh after Edit" in the IMAGE menu.
Now repeat the same for the alpha channel (ALPHA menu --> "send to editor"). Only this time, flood fill the whole image with 100% black (0,0,0 color) and then type on it in 100% white (255,255,255). Save the file when you're done, return to DXT2BMP and run "refresh alpha" in the ALPHA menu.
Now save the DDS file in DXT2BMP, return to RW_Tools, and convert the whole thing back to its original TgPcDx. If you didn't exit RW_Tools while you were editing the image, it will find the correct files automatically after you click on convert to tgpcdx.
Hope this helps.