Niantic's Pokémon Go developers do their best to bring any material from the original games to mobile devices. Sometimes, they have to be a little creative for the mechanic to work on a mobile device, and the Pokémon, Inkay, is one of them. Traditionally, when you wanted to evolve this Pokémon, you had to flip your device. When you want to do this in Pokémon Go, you have to do the same thing. However, this feature requires a gyroscope. If your phone can play Pokémon Go and it doesn't have one, can you evolve Inkay?
Unfortunately, it's not possible to evolve Inkay if your phone doesn't have a gyroscope. You will be left out on that front, and from what we can tell, there is no way around it. The only thing we've been able to think of is to log into a friend's or family member's mobile device and use the phone's gyroscope.
Alternatively, you can always ask a friend to evolve Inkay into a Malamar and then try to trade it with you. The downside to that plan is that every time you trade a Pokémon between trainers, the stats change. It's best to try using your Pokémon Go account on a different device rather than swapping it with another player.
Hopefully, Niantic will find a way to give players a better process for this.