Einstein believed gravity isn't really a force in the same sense as the other three fundamental forces, but a property of space-time. That seems to be a problem because it causes some kind of "incompatibilty" with the others. Trying to reconcile those, efforts has been made to create quantum gravity theories, that view space itself as quantized and in this direction you find strings and quantum loop gravity.
Disclaimer: I'm no expert, just parroting what I found in Wikipedia.
IANAP either but as far as I understand the incompatibility is not on wether gravity is a force or not (that's just a way to interpret the equations). The issue is that all attempts to quantize gravity have failed so far (due to the specific mathematical properties of the gravitational field) and QFT would require gravity to be quantized to interact with other quantum fields.
Disclaimer: I'm no expert, just parroting what I found in Wikipedia.