I wouldn't say that WB are making this film in response to GotG, because, like you said, it's been in development for a while. I do, however, think that GotG prompted them to put more money into it and go after big names. Back when we first heard talk of this film, I believe it was intended to be a smaller, lower budget film. I think that was when DC still had the mentality that that only big names like Superman and Batman could draw in huge crowds. However, I think GotG proved to everyone that you could take characters most people have never heard of and make a really successful movie with them, as long as said movie was good and featured some recognizable faces (and voices).
I think, at that point, WB got more confidence in Suicide Squad, and began fast-tracking it, pumping more money into it, and targeting some big names. I mean, it is a bit unusual that out of DC's entire library of characters, Suicide Squad is the first film that will hit theaters after BvS, when much more recognizable characters also have films in the works (Wonder Woman, The Flash). But I think it's a good move, because it helps WB with their world-building concept and yet manages to do something different than what Marvel is doing by having a film with mostly villains come early on in the process.