Check out MikroE, they have a Pi plug-on board (or 'shield') and various daughter boards that plug on top, including a GSM one. But it is not technically a 'HAT'.
I don't think anyone has made a 3G/4G HAT.
A cheaper/easier approach would be to plug in a 3G/4G USB dongle.
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With the PiAnywhere HAT module for the Raspberry Pi you can take your little computer anywhere. This powerful HAT gives you 4G, 3G or GSM mobile data, GPS, Bluetooth and a rechargeable power source for your Raspberry Pi.
Tony, just to say, I checked these boards and I made a couple of sums... IMHO the pricing is really high - despite that the product seems well done. There are 15-20$ cheaper solutions that I think are more useful for makers (just not plug-and-play) but can also be more intriguing and not so difficult to implement.
At this purpose may fit your needs and are easy to connect to the PI (I have done it with a PSoC microcontroller) via serial the Quectel L70 or L80 modules or the SIM800 based GPS units.
