Here is an email I got from an actual employee of GT:

If the bike frame was manufactured at the Santa Ana CA facility, the first three or four digits of the serial number should be letters abbreviating the model or size of the frame such as 'XL', 'XXL', CR24, or something like that.

The next part of the serial number is eight numeric digits. The first four numbers should be a two digit month followed by a two digit year.

Bicycle serial number locatons 10-top 4-top 3-front 9-front plate 12-left 1 -bottom 11 -lower back left 2-right 5 -left. Gary fisher- gazelle- 1 gitane-5. Welcome to Rickenbacker's serial number decoder page. Rickenbacker serial numbers have been issued in a number of formats. To the right and below, you will find some images that will hopefully help you to locate the complete serial number on your instrument. Please be aware that some instruments manufactured in the late '50s cannot be. I recently sold a Gary Fisher frame on Ebay and the buyer is disputing the frame size as I listed it (I believe he is measuring the frame center-center instead of center-top). The only proof I have that the frame size is indeed the size I listed is a bill of sale with the serial number of the. The bicycle looks good, may have minimal cosmetic (only) blemishes on the frame, fork, components, parts and/or accessories. The bicycle may require minimal service adjustments without the need of replacing any parts. The range of Serial Numbers is 2999497 to 3219847. Find it on a plate attached to the frame under right front fender. Since DMV's used one or the other for a registration ID, look for a number within these ranges. Dave Corbin can provide much more info, but first you have to find the number.

The last four digits are the serialization digits 0000 to 9999 depending on what number the frame was of the years production.

If the frame was made overseas in Taiwan, it gets more complicated. If the number starts with a K, the frame was made by Kinesis.

Fisher Serial Number Lookup

That's the only one I remember off the top of my head.

There were at least four companies in Taiwan that made the GT lines :

Robinson, Dyno, Powerlite and Auburn included)

The first digit would be a letter designating the factory that made the frame. There might also be a second letter digit specifying the factory that assembled the bike, if applicable. The model and year of production was also coded by a letter, so you won't be able to spot a two digit month and two digit year.
Before I left, I was writing a specification to standardize all of the serial numbers because GT was starting to send most of its US production to other US fabrication shops.
The information might also come in handy for deciphering other makes because the factories in Taiwan make bikes for a lot of different companies.
I am tinkering around with the idea of starting to make my own frames, kind of on a 'one-off' basis. I am also teaching welding at a local community college.
By the way, did I tell you that the guy ran GT's in house fabrication for the last ten years (he recently retired) was none other than Gilbert Axt. Who? Do you remember Race, Inc.? That was Gilbert.
He also did the original PK Ripper for SE Racing. History, man.
It might be a really cool thing if I can track down Gilbert now and interview him for you.
THAT is the kind of stuff that belongs in the BMX Museum!

Gary Fisher Serial Number Decoder

Later. I'll write you when I get a chance. Dan Garcia

Gary Fisher Serial Number Decoder Lookup

and here is the updates, thanks to love thegoose! (Michael S)
well we get a good bit of gt serials that could be identified by the owner, if they only had the tips that some of us have learned, so i will post what i know, and anyone with more knowledge can add to it..so here we go.
here is an example to go by. on the dropout you have 'M1' followed by '08922198'..this one is simple.
M1 stands for 'mach one' that is your model gt.
it can also be a 'I' for interceptor and so on.
next on the serial is '0892' this is easy to decode too,
08 means 8th month, which would be august.
then you have '92' next in the lineup, this would be 1992
the last 4 digits, are your build number, this one would be the 2,198th one built.
Here is another example. if your serial is KGCG2297 this one gets a little complicated. but still easy once you learn the code to it.
the first letter is who made the frame itself, in this case it would be 'Kenisis', an overseas company.this could be replaced by another letter but at least we know what that one is there for.
next is the letter 'G'..this isnt always there, but if so, this is where it was assembled, (ever notice the built in usa sticker on your gt or dyno?) this is what that letter is there for. it wasnt made here, but was built here.
next is the letter 'C'..C is the 3rd letter in the alphabet, and march is the 3rd month, so march is the build month.
next is the letter 'G' this is the year. G is the 7th letter in the alphabet,
so this would be a 1987.
here is one more..your serial resembles this:f6030578.
to start, if you have a 'baseball' looking stamp behind your serial, this frame was made overseas...no its not a pacific, so dont cry, or get mad...and lets continue.
on this serial, the first number is the last digit of the year, so 6 stands for 1986
next is '03'....this is the third month..which is march.
so you have a gt built in march of 1986.
but still use the museum as a guide to id the frame, cause sometimes you can find the year, but getting a posotive id on the model isnt possible without knowing which style frame you have.
this doesnt apply to new gt's, but hopefully some people with info on those will chime in. this isnt always going to match your serial, you may have a 'fake'..its not likely but it happens. also on earlier models this can differ. this is for the 2 most popular style serial's i see on here. also, common sense tells you that if you have a suspected 'gt' frame, look it up and compare it in the musuem, knowing which style frame you have will help with the process of id'ing your bike with the serial, especially if the serial has some damage that prevents you from getting the full number.
hopefully this bit of info can be used for everyone, and this will save you from those late nights sitting around waiting for a response to your post looking for an answer to the idendity to your gt...although i do feel smart and proud when i can answer these posts. well, thats it for me, if anyone else has info on serials or if you see a mistake in this guide, tell me so i can edit it. happy holidays people.