It only seems like a foregone conclusion if the Nationalists remain united. Had Franco not taken charge and had the other leaders for the Nationalists not died so early in the struggle, we could have seen a very different outcome. The Nationalist side had many factions that were also ideologically opposed to each other, with their hatred of the socialists and communists being their one big uniting point. Carlists and Falangists were not exactly buddy-buddy; their views on state control differed drastically and would have likely resulted in clashes had Franco not reigned them in.
And while the Republicans had obvious differences, they were actually in a fairly good position at the start of the civil war and held more critical territory that aided their war effort. Had they not made some fateful decisions and somehow put aside their differences early on they may have had a shot at winning, however slim the possibility was.
In Spanish there are several, among the prominent ones that were published:
"En el día de hoy" (On this day), by Jesús Torbado (1976). It postulates that the Republic wins the Civil War on April 1, 1939, and the title derives from the part with which Franco announced the end of the war.
My main criticism would be of the fact that it's basically "OTL in reverse", swapping losers and winners but keeping everything else pretty much the same. The story ends when Hitler decides to launch Operation Felix in 1940, so we never know how the story ends.
"El Coleccionista de Sellos" (The stamp collector), by Cesar Mallorquí (2020). It begins with what seems like a routine police procedure in a Republic that is about to win the war, but it is soon revealed that there is much more to the search for the mysterious postage stamps...
My main criticism is that the origin of the entire plot is based on the fact that the creators of the stamps that allow letters to be sent through time are, to put it plainly, imbeciles. Okay, I can believe one of them mistakenly sent a set of "time stamps" back in time. But in that case, it would have been much easier to send a letter to that person with a message of "this is a bad idea, you better not do it" than to constantly send letters to the past to try to manipulate people from 1939 to do your job for them. Not to mention that we are never told how that system is supposed to work in a time before the invention of mailboxes, or in a post-postal mail time, like our 2020, where it is increasingly difficult to find a postbox on the street. . Not to forget that the ending basically implies that all the efforts of the protagonists will simply be undone in the next timeline.
The others that exist are basically minor short stories (some quite absurd, I remember one in which the Republic wins because they somehow built an atomic bomb) or they focus more on "what is Spain like now that the Republic won", without pay the slightest attention to anything outside the immediate environment of the protagonists.
