Pulsars are fascinating stellar objects. Their nature being a celestial clock make them extremely sensitive probes to tiny perturbations in spacetime. In this talk, I will introduce two applications of pulsar to probe our theory of gravity under extreme physical conditions: Detecting low-frequency gravitational waves with a pulsar timing array (PTA), and measuring the properties of the supermassive black hole (Sgr A*) in the center of our Galaxy. I will show the most recent results obtained by the European PTA and the International PTA, where we have detected a common red-noise signal among multiple PTA pulsars. This signal may be the first evidence towards a final detection of the gravitational wave background. I will also present an overview of the currently ongoing pulsar searches towards the Galactic Center. This will include the first effort of such using the Atacama Large Millimeter Array and the Event Horizon Telescope.