Stereotactic body radiotherapy (SBRT) of the lung has become a standard of care for early-stage inoperable non-small cell lung cancer (NSCLC). A common strategy to manage respiratory motion is gating, which inevitably results in an increase in treatment time, especially in irregularly-breathing patients. Flattening-filter free (FFF) beams allow for delivery of the treatment at a higher dose rate, therefore counteracting the lengthened treatment time due to frequent interruption of the beam during gated radiotherapy. In this study, we perform our in vitro evaluation of the dosimetric and radiobiological effect of gated lung SBRT with simultaneous integrated boost (SIB) using both flattened and FFF beams. A moving thorax-shaped phantom with inserts and applicators was used for simulation, planning, gated treatment delivery measurements and in vitro tests. The effects of gating window, dose rate, and breathing pattern were evaluated. Planned doses represented a typical conventional fractionation, 200 cGy per fraction with SIB to 240 cGy, flattened beam only, and SBRT, 800 cGy with SIB to 900 cGy, flattened and FFF beams. Ideal, as well as regular and irregular patient-specific breathing patterns with and without gating were used. A survival assay for lung adenocarcinoma A549 cell line was performed. Delivered dose was within 6% for locations planned to receive 200 and 800 cGy and within 4% for SIB locations. Time between first beam-on and last beam-off varied from approximately 1.5 min for conventional fractionation, 200/240 cGy, to 10.5 min for gated SBRT, 800/900 cGy doses, flattened beam and irregular breathing motion pattern. With FFF beams dose delivery time was shorter by a factor of 2-3, depending on the gating window and breathing pattern. We have found that, for the most part, survival depended on dose and not on dose rate, gating window, or breathing regularity.