Listen carefully to the ones who tell stories of woe about California bikes that owe back registration fees.
But really, why would you want to turn a 1977 R100/7 into something it is not? A GS. If you want a GS buy a GS. I think it would be cheaper. I could see putting knobby tires or something on the /7 but I think calling it a GS involves a whole lot more. Maybe what you are thinking of is what I've seen called "dual sport".
There's lots of info and projects on Adv Rider about doing this to the Airhead bikes. Some of these look pretty good, the builders are talented and they have the tools and money for some extra work not seen everyday. But you should understand that the majority of these projects, after the spending of hundreds of dollars, is worth lest that the bike was in stock fashion.
Good Luck w/ it. I hope you get the bike. Try riding it the way it was built for a little while at least. Then the Old Farts like myself won't pick on you so much.
PS; What over bikes do you own or have owned?