Galdós and Lorca seem recent enough that any translators would probably not have died within most public domain limits. As for Bécquer and Martí, the problem may well be obscurity outside of Spanish-speaking regions/specialty lit classes (I'd never heard of any of them besides Lorca, and that mainly because of the Spanish Civil War; thanks for letting me know).
Also, PG seems to be very heavily dependent on English-speaking proofreaders, which may skew the number of non-English language texts available.
The semi-associated spinoff
Projekt Runeberg site has by far better selection of Nordic works, both in original and translation, simply because the people working on it are mainly from the Scandinavian countries and therefore know the language they're proofreading and have the ability to select/read/find the more obscure texts to add in the first place.
So possibly there might be a dedicated Spanish-language project which does the same thing. I certainly hope so.