Accurate tracking of lesions within or near the heart presents a unique challenge due to cardiorespiratory motion during stereotactic radiotherapy. Mitigating this motion is essential to limit the risk of radiation-induced complications. Here, the feasibility of real-time prospective myocardial landmark-tracking with Gaussian Processes from few k-space readouts is demonstrated on the 1.5T Elekta Unity MR-linac. Retrospective and prospective phantom and in-vivo experiments were conducted in 2D, and preliminary in-silico experiments were done to demonstrate the feasibility in 3D. Results indicate high accuracy, and thereby great potential for real-time 3D myocardial target tracking for cardiac radio-ablation.