We investigate Lorentzian quantum gravity coupled to a template matter sector with gauge fields, scalars and fermions. In the absence of quantised gravity, the matter sector by itself is renormalisable, but UV-incomplete. Provided quantum gravity offers an asymptotically safe UV completion, we determine the photon and scalar two-point functions in the presence of gravitational fluctuations, and show that both possess a K"all'en-Lehmann spectral representation. Our results are achieved using functional renormalisation adapted for theories in Lorentzian signature. We explain why and how interactions with gravity modify both the infrared as well as the ultraviolet behaviour of matter spectral functions. We further determine the corresponding form factors on the level of the quantum effective action. Limitations and extensions of our study are discussed alongside implications for particle physics and unitarity of quantum gravity with matter.