Recently stayed at the HP Frankfurt airport again during a short 1.5 day stopover. The hotel is a decent option for a short stay. Their breakfast buffet has a good selection of warm and cold offerings. Overall our short stay was comfortable, room was clean and it was easy to get around.
My previous stay was just after it had opened and there was no metro station but the hotel did have a shuttle. Now the shuttle is not offered but the S-bahn nearby is quite convenient for getting around.
As another person mentioned upthread, it’s a single stop from the airport to Gateway Gardens. You just need to purchase a short trip ticket which is under €2 one-way. The hotel is just a few feet from the exit of the metro station. If you are interested in exploring Frankfurt, there are regular S-bahn options to the main station and other stops and takes 15 mins or so.
During our stay, the ticket machine at the Gateway Gardens metro didn’t accept any Euro bills (just coins and cards) and there’s no other option to purchase tickets. There were a couple of folks who only had bills with them and were scrambling to figure out how to get their tickets, so something to be aware of.
There’s also a Rewe grocery store within a 5 min walk (next to the Moxy hotel) as well as some cafes and restaurants nearby.