Ok:
Try a few things here...try renaming your bigo video file to pgr3.wmv and use the attached in your gamedb.ini file:
[Game]
Title=Project Gotham Racing 3
Developer=Bizarre
Publisher=MS Game Studios
ConsoleTypeFlags=0
GenreIndex=3
Description=Project Gotham Racing 3 expands on features racing gamers have grown to love. A truly next generation game both online and off, it is the ultimate combination of breathtaking speed and dynamic style.
FullDescriptionFile=pgr3.txt
IconPath=game:\images\gtn-pgr3.png
ImagePath=game:\images\mta-gm-pgr3.png
BannerPath=game:\images\ban-gm-pgr3.png
VideoPath=game:\videos\pgr3.wmv
RatingIcon=app:\ESRB_E.png
RatingText=
Leave all the data from the orignal image directory. Also, when you burn your disc, make sure you finalize the disc (no further writing possible)
If this still does not work, your wmv file is probably not compatible with the 360. Use videora to convert or try using a demo 720p video from wmvhd.com.
Hope this helps...you can fool around from there if it works.
Here is an example of a gamedb.ini file from a pron site I made for a friend

[Game]
Title=Candi and Renee
Developer=Poomobile (720p)
Publisher=Poomobile Productions
ConsoleTypeFlags=0
GenreIndex=1
Description=More HD porn from Snatches.com!
FullDescriptionFile=snatches11.txt
IconPath=game:\images\candiicon.PNG
ImagePath=game:\images\candiimage.png
BannerPath=game:\images\candibanner.png
VideoPath=game:\videos\Snatches-candirenee-UltraHD.wmv
RatingIcon=app:\ESRB_M.png
RatingText= xxx