Hard to say how good Gerrard is doing at Rangers domestically because Celtic are their only competition and Lennon is not a top coach, but I have been really impressed with their results in Europe, he has really turned them into a competitive team at Europa League level and that is a good sign.
If Klopp leaves then Gerrard would be a good safe choice as he would get plenty of time from the fans to make it happen and I think he would do good job, but Klopp is an exceptional manager and Gerrard has a long way to go to prove he is in his League so whatever happens I think it will be a step down to for them in the short to medium term.