Do you mean that these people are going to stand in a way that each makes up one "dot" (pixel?) in the picture, and so when viewed from a height the 6000 people will form a portrait of Gandhi? If that's what you're trying to say, this is a rare enough idea that there's no single English word to describe it, you'd have to explain it with a sentence or two.
If you're trying to write a few words that would convey EITHER the idea of people drawing a portrait or of people's bodies making up the image, you could just say "human-made". Or you could leave out the word "human" completely and just say "the largest portrait of Gandhi ever made". As you say, we generally assume a portrait is made by humans unless otherwise specified. (If a monkey drew a portrait or we found a portrait on Mars made by aliens, you'd need to specify that.)
If you're writing a headline, headlines are expected to be brief. You could make the headline ambiguous and then explain in the body of the article. Like, "6000 people to form huge portrait of Gandhi", then explain in the article.