The instructions are different depending on what model of calculator it's being run on, but most (not all) Texas Instruments graphing calculators from the past 16-ish years can run GameBoy games.
For the Ti-nSpire/CAS/CX, it requires two things. NDless, the jailbreak application, and GBC4nSpire, the GameBoy emulator. Just get the correct version of NDless for your calculator by following this guide: (Select your calculator's operating system version)
Note: OS version 3.9 requires plugging the calculator into a computer every time you restart it, so I wouldn't recommend upgrading to 3.9 if you can help it.
Once that's done, you can put third-party apps on your calculator. In this case, we'll want GBC4nSpire. Using the nSpire Computer Link app that the previous step had you use, drag the "GBC4nSpire.tns" file from inside the "GBC4nSpire.zip" folder onto the calculator.
The final step is to put the GameBoy ROM on the calculator, but one thing needs to be changed beforehand. Add ".tns" to the end of the file. In the case of Rulent Tower, it will read "rulenttowerv041.gb.tns" when it's ready. Now, it can be put on the calculator with the nSpire Computer Link app.
Once everything is ready and NDless is running, open GBC4nSpire and select Rulent Tower.
For other calculators, such as the Ti-84+ Silver Edition, just Google "how to play GameBoy games on (calculator name)."