We demonstrate the development of anti-resonant hollow core fibers functionalized with nanodiamonds and their application to ODMR-based and microwave-free magnetic field gradiometry. The diamond particles were uniformly deposited from a solvent suspension on the inner surfaces of two 50 cm long hollow-core fiber sensors. This enabled efficient excitation of the diamond fluorescence using 532 nm light, as well as collecting the nitrogen vacancy color center red fluorescence to the hollow fiber guided modes, which facilitated fully guided mode operation of each sensor with 400 nT/sqrt(Hz) sensitivity. Two fiber probes in a differential arrangement provided magnetic field gradient measurement with a 35 mT dynamic range without the presence of microwaves. Specific applications, like the control of magnetic switching in spintronics computing, could benefit from accurate and microwave-free readout modalities for mT-scale magnetic fields. The demonstrated hollow core fiber probes fulfill these requirements. They combine the robustness and the possibility of macroscopic separation between the scanned target and the readout location due to the guiding of the NV fluorescence, while maintaining a dynamic range that matches state-of-the-art single-crystal diamond-based sensors.