Mutations in the gatekeeper residue of kinases have emerged as a key way through which cancer cells develop resistance to treatment. As such, the design of gatekeeper mutation resistant kinase inhibitors is a crucial way forward in increasing the efficacy of a broad range of anticancer drugs. In this work we use atomistic simulations to provide detailed thermodynamic and structural insight into how two inhibitors of cSrc kinase, namely, a commercial drug and type I kinase inhibitor Dasatinib and the type II inhibitor RL45, respectively fail and succeed in being effective against the T338M gatekeeper residue mutation in the kinase binding site. Given the well-known limitations of atomistic simulations in sampling biomolecular systems, we use an enhanced sampling technique called free energy perturbation with replica exchange solute tempering (FEP/REST). Our calculations find that the type I inhibitor Dasatinib binds favorably to the wild type but unfavorably to T338M mutated kinase, while RL45 binds favorably to both. The predicted relative binding free energies are well within 1 kcal/mol accuracy compared to experiments. We find that Dasatinib's impotency against gatekeeper residue mutations arises from a loss of ligand kinase hydrogen bonding due to T338M mutation and from steric hindrance due to the presence of an inflexible phenyl ring close to the ligand. On the other hand, in the type II binding RL45, the central phenyl ring has very pronounced flexibility. This leads to the inhibitor overcoming effects of steric clashes on mutation and maintaining an electrostatically favorable "edge-to-face" orientation with a neighboring phenylalanine residue. Our work provides useful insight into the mechanisms of mutation resistant kinase inhibitors and demonstrates the usefulness of enhanced sampling techniques in computational drug design.