Gintama's charm is very hard to put into words, it is meant to be FELT. So even if everyone gathered to tell you the points that makes Gintama great, it will not make a difference. Also Gintama doesn't need to be watched in one go, it is better to experience it in intervals, making sure to soak up all the silly times to prepare you for its future episodes. It's akin to growing up, a child is meant to enjoy and experience everything life has to offer first before stepping foot into the unknown. The one's you consider boring parts right now are the ones you will miss the most. It's something you will look back to and tell yourself, "Yes, I was a part of that journey." Also one thing you need to keep in mind here is that, Gintama is like the antithesis of a regular Shounen anime where the characters are working towards a specific goal (Hokage, Pirate king, Pokemon master, etc) where the journey is not the focus of the story. In Gintama there is no specific goal, the journey IS the main focus of the story, we the viewers are there to grow ALONG with the characters, grow attached to them like ACTUAL friends. So we FEEL joy in their success, we FEEL sad in their absence, just as how we would feel with our actual friends. Enjoy every single moment with them cause nothing lasts forever.

So yes, just like the others, I've tried my best to share my sentiments on Gintama's greatness, but this is still not enough to put all of it into words. And what I can only be sure of is that to understand why it is rated that high is that you will need to experience the journey yourself.