In many cases, gatekeeper genes encode a system of checks and balances that monitor cell division and death. When tissue damage occurs, for example, products of gatekeeper genes ensure that balance of cell growth over cellular death remains in check.  In the presence of competent gatekeeper genes, mutations of other genes do not lead to on-going growth imbalances.
Mutations altering these genes lead to irregular growth regulation and differentiation.  Each cell type has only one, or at least only very few, gatekeeper genes.  If a person is predisposed to cancer, they have inherited a mutation in one of two copies of a gatekeeper gene.  Mutation of the alternate allele leads to progression to neoplasia.
Historically, the term gatekeeper gene was first coined in association with the APC gene, a tumor suppressor that is consistently found to be mutated in colorectal tumors.  Gatekeeper genes are in fact specific to the tissues in which they reside.
The probability that mutations occur in other genes increases when DNA repair pathway mechanisms are damaged as a result of mutations in caretaker genes.  Thus, the probability that a mutation will take place in a gatekeeper gene increases when the caretaker gene has been mutated.
Apoptosis, or induced cell suicide, usually serves as a mechanism to prevent excessive cellular growth.  Gatekeeper genes regulate apoptosis.  However, in instances where tissue growth or regrowth is warranted, these signals must be inactivated or net tissue regeneration would be impossible.  Thus, mutations in growth-controlling genes would lead to the characteristics of uncontrolled cellular proliferation, neoplasia, while in a parallel cell that had no mutations in the gatekeeper function, simple cell death would ensue.