its not the transmission light it sounds like the engine management light, when on the car can go into 'limp home' mode, and restrict you to 4,000 rpm.
the reason for it coming it is impossible to guess here, but when it comes on a fault code will be stored in the ecu. this code will tell you what the reason for the light coming on is, and will store in the ecu even after the light go's off for retrieval later.
the easiest way to get this checked is by taking the car to a vauxhall dealer and getting them to plug it in to their Tech 2 machine, which is a hand held computer that plugs into the car. obviously they will charge you for this, could be around £40 (depends on the individual garage) but you will have some peace of mind and know what the fault was.
there is a way you can do this yourself, by using a paperclip, and i have done this many times for my own vectra v6, and for other peoples cars. However, i am not going to explain that here as the info is available on the internet, and seeing as i dont know what engine, what year, etc, you risk damaging the car if you do it wrong.
hope that helps :)
rufe, February 2005
