Talk about a blue Spain...
I didn't know that Gomera was a red island in an ocean of blue Canaries.
It was German hippie land in the early-mid 1980s, especially Valle Gran Rey, and later became an eco-tourism site for its mountain forests. That may have influenced locals (especially the younger part of them).
It might be a plausible explanation, but the strength of socialists in La Gomera has more relation with Casimiro Curbelo. As Hash said, it was president of the Cabildo since 1991 and previously mayor of San Sebastián (the island's capital, 1983-1986). Curbelo has governed La Gomera with a paternalistic style (some people say he's a
cacique). There was a time in which burials were paid by the Cabildo.
Hippies in the smaller islands in the Canaries were tending to live in their own ghetto (language and cultural barriers) and I think they had very little influence in local population. From my own experience, I think people in the island is a bit distrustful and probably not so permeable to external influences in the past. Eco-tourism development is relatively recent. On the other hand, Valle Gran Rey was traditionally a CC's (center-right, regionalist) enclave in the red island.
I met several old hippies and other foreigners living in La Palma, a place that I'd say it's socially conservative like La Gomera (it has eco-tourism and beautiful mountain forests as well). There are people totally integrated with the local population, but others live segregated. Once I visited with some friends a German woman who was living about 20 years in a beautiful rural home in the south of La Palma and she knew very few words in Spanish language. There are some hippy enclaves in the traditionally isolated north of the island (Garafía, Las Tricias).
