The gTAA structure is designed to organize the test automation process into distinct layers, each with specific responsibilities:

a. Test generation layer (3): This layer is responsible for designing test directives that allow configuring the algorithms used to automatically produce the test cases for a given model of the System Under Test (SUT). It involves the creation of models from which test cases can be generated.

b. Test definition layer (4): This layer allows the definition and implementation of test cases and data by means of templates and/or guidelines. It’s where test cases are articulated and documented.

c. Test execution layer (2): This layer allows the automated test scripts on an abstract level to interact with components, configurations, and interfaces of the SUT. It’s the practical application of the defined test cases against the SUT.

d. Test adaptation layer (1): Although not explicitly mentioned in the question, the test adaptation layer is implied as the fourth layer. It acquires all the necessary resources before each test and releases all after the run to avoid interdependencies between tests. This ensures that each test is self-contained and does not affect subsequent tests.

References = The information provided is based on the ISTQB Test Automation Engineer syllabus and the principles of the gTAA structure as outlined in the ISTQB certification materials123.