Quantum optical metrology aims to identify ultimate sensitivity bounds for the estimation of parameters econded into quantum states of the electromagnetic field. In many practical applications, including imaging, microscopy, and remote sensing, the parameter of interest is not only encoded in the quantum state of the field, but also in its spatio-temporal distribution, i.e. in its mode structure. In this mode-encoded parameter estimation setting, we derive an analytical expression for the quantum Fisher information valid for arbitrary multimode Gaussian fields. To illustrate the power of our approach, we apply our results to the estimation of the transverse separation between two point sources observed with a diffraction-limited imaging system. In this context, we determine the ultimate resolution provided by mutual coherence and squeezing of the sources.