Yes it is. In fact all GT80's were availible in a "street legal" model, and have been making them since the GT80 A, B and C series. if you are planning to make a yamaha GT 80 street legal, you can order parts such as lights, and other electrical components on ebay or other motorbike dealers.
Street-Legal - album - was created in 1978-04.
Yamaha recommeded 20/1
The 2K9 motor is either a 1978 IT250E or a 1978 IT400E depending on if it is a 250cc or 400cc.
Heat in the Street was created in 1978.
1978 dt4
20w50
A Special Sesame Street Christmas - 1978 TV was released on: USA: 8 December 1978
Street Hassle was created in 1978-02.
Street Action was created in 1978-02.
Street Player was created in 1978-01.
I found parts scematics to view for a 1978 Yamaha Chappy here: http://www.cmsnl.com/yamaha-lb50pe-19781979_model8675/partslist/B-01.html
That would be a 1978