Thank you for sharing your update. I am glad to hear that the GMAC driver is now working for you.
Regarding your additional questions:
1. The main reference for configuring and using the driver is the Eth_43_GMAC User Manual, which is included in the RTD installation, e.g.: C:\NXP\S32DS.3.6.6_RTD701\S32DS\software\PlatformSDK_S32K3\RTD\Eth_43_GMAC_TS_T40D34M70I1R0\doc\RTD_ETH_43_GMAC_UM.pdf
I also recommend reviewing the corresponding Eth_43_GMAC Integration Manual (RTD_ETH_43_GMAC_IM.pdf), saved in the same directory, and the available GMAC example projects.
2. Whether Eth_43_GMAC_ReleaseRxBuffer() must be called depends on the EthCtrlReleaseResourceAfterReception configuration parameter.
If automatic release after reception is enabled, the driver releases the RX resource after EthIf_RxIndication() returns and you should not release it explicitly.
If automatic release is disabled, the received buffer remains allocated after EthIf_RxIndication() returns. In that case, the application must eventually call:
Eth_43_GMAC_ReleaseRxBuffer(CtrlIdx, RxHandleId);
This mode is useful when the application needs to retain the received data for further processing. However, the buffer must be released after the application has finished using it. Otherwise, the available RX buffers will eventually be exhausted.
Please also note that the received Payload pointer should not be used after the corresponding buffer has been released.
3. The Eth_43_GMAC User Manual is also the primary reference for the individual configuration parameters. In addition, S32 Configuration Tools displays a description for many parameters when the corresponding setting is selected or hovered over. For hardware-level behavior, such as MAC filtering, DMA operation, MTL queues, timestamps and interrupt status, please refer to the GMAC/EMAC chapter of the S32K3 Reference Manual.
As a practical starting point, I recommend referencing GMAC examples provided by the RTD package. There are also functional lwIP examples on the page:
https://community.nxp.com/t5/S32K-Knowledge-Base/S32K-Examples/ta-p/1108990
Please do not hesitate to ask for support in case you observe any issue. A dedicated thread for each topic is preferred. It helps keep us clarity of cases for other users.