A persuasive speech is simply a speech in which you are trying to persuade someone of something. In your case, you want to give a speech that persuades people that technology is making people gain weight, so yes, that would be considered a persuasive speech. You should order your speech so that you first go over why you think technology is making people gain weight. Then, once you have stated your argument, you can move on to stating your ideas about what can be done to solve the problem. That's about all the advice I can give you without knowing any of the particulars of your speech.