Shows like Getter Robo or Zambot feature vehicles that (slightly transform) combine to form a giant super robot. What was the first anime or manga to use the combining tactic and was there an inspiration for this?
In most likelihood it was Getter Robo which first appeared in 1974. The story goes that Go Nagai came up with the novel idea of machines combining together to make a single giant robot. He communicated this idea to his friend and fellow mangaka Ken Ishikawa, who went on to create the Getter Robo series (which is why Nagai is often credited as a co-creator, if not sole creator).
On the other hand, Himitsu Sentai Gorenger (the first Super Sentai show) aired in 1975 and didn't have any giant robots, combining or otherwise.