I've restored cars professionally and I've worked at a lot of shows. In general people go for cars that they remember from their childhood or ones that they just don't see anymore. Pre-WWII cars used to be the thing to get, but the people who remember them have mostly died off so they've dropped in value while muscle cars have skyrocketed.

At the last show I was at there were lines of serious Detroit metal from the '60s but all the attention was on a BMW Isetta and a mid-80's Toyota truck converted to electric power. The former was a joke when it was released and the later as common as a department store bike, but in an unusual form.

I bought a Free Spirit 10-speed recently and have had a lot of interest in it from a friend because he had one exactly like it as his first bike. There's nothing remarkable about the bike itself, but it comes with a lot of memories. Likewise, someday someone will pick up one of these GTs and reminisce when it was more than just a label slapped on a generic bike.