Editor for bitmap fonts from Adrafruit-GFX Library and TFT_eSPI for Arduino
With this program you can see bitmaps of letters and include or edit one.
For the GLCD font, you can use it to see and create a new one, but you can place the line of bitmaps into the file manually.
You can see some letters added to .h font files in my project TTGOWeatherStation.