Sorry, the article is in Spanish, I've not found an English article about this strike.

From the article:

Almost silently and like a living tide, the call for the Strike for Palestine on September 27 , called by the unions CGT and Solidaridad Obrera (SO), has been gaining strength. Having launched the official call towards the end of August, little by little other non-majority unions (COS, SAT Granada, Co.bas...), organizations, groups from various sectors of society have shown their intention to subscribe to it. Dozens of posters in various territories of the Spanish State are already circulating, referring not only to continuing the strike but also to making it a day of active struggle throughout the day.

The General Strike has not the support of the two mayor Spanish unions (CCOO and UGT)

The manifesto, posters, support/joining form for associations and calls for picket lines/demonstrations in Catalonia (the part in Spain) are accessible from this website: https://defensempalestina.org/ (in Catalan)