I am pretty sure it is short for Gundam Plastic - term used for modeling kits released by Bandai. There may be a more accurate and complete answer out there... I was actually searching for the history of the word and the possible coin date of the phrase when I found the question here.
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it is gundam plastic model in short form
No sorry.
Actually you are referring to the Gunpla BB Senshi Line of model kit figures. These line I believe was not made into an anime series but i think there is manga for it.
In 1979, Yoshiyuki Tomino created Mobile Suit Gundam, a futuristic-like TV series. The first time it aired, it did not do well. Then, in some Star Trek-like coincidence, the popularity of it soared after the end of the original run. A huge metaseries was born from that TV show.