There are several things regarding ESP-Now is not setup correctly.

1. For ESP-Now Receiver, it needs to be in AP mode withWiFi.mode(WIFI_AP), and make sure it doesn't connect to WiFi withWiFi.disconnect().
2. For ESP-Now Transmitter, it needs to be in STA mode (this is done correctly) and itMUST NOT connect to WiFi, so addWiFi.disconnect() right afterWiFi.mode(WIFI_STA).
3. You don't need to register the callback repeatedly in the loop, the lineesp_now_register_send_cb(OnDataSent); should be moved tosetup(). 4. For the sendData(), remove theuint8_t result = and theif statement, the sendData status is provided in the callback function.
4. For thereceiveMAC[] address in the transmitter need to either match the actual MAC address of the receiver ESP8266 or theLocally Administrated MAC Address, the advantage of using the locally administrated MAC address is that a) you don't need to write a sketch to find out the MAC address as shown in the other answer; b) when you change swap ESP8266 receiver with another new one in future, you don't need to re-flash your transmitter to change the MAC address.

The following is the example code (without the GPS code) for the ESP8266 ESP-Now transmitter and ESP-Now receiver, with the example of using a locally administrated MAC address.

ESP-Now Transmitter

#include <ESP8266WiFi.h>#include <espnow.h>typedef struct gps_message_s {  float latitude;  float longitude;  float altitude;} gps_t;// *** the following variables need to match the receiver settings ***uint8_t receiverMAC[] = {{0x82, 0x88, 0x88, 0x88, 0x88, 0x88};gps_t myData;const unit8_t channel = 1;// **************************************************************unsigned long sendStartTime;void OnDataSent(uint8_t * mac, uint8_t sendStatus) {  unsigned long sentEndTime = micros();  Serial.printf("< Device: %02x:%02x:%02x:%02x:%02x:%02x\n",                 mac[0], mac[1], mac[2], mac[3], mac[4], mac[5]);  Serial.printf("< Status: %s\n",                  (sendStatus == 0 ? "Success" : "Failed"));'  Serial.printf("Round-trip in uS: %4lu, ", sentEndTime - sentStartTime);}void setup() {  Serial.begin(115200);   WiFi.persistent(false); // turn off wifi persistent  WiFi.mode(WIFI_STA);    // ESP-Now transmitter must be in STA mode  WiFi.disconnect();      // Do not connect to WiFi  if (esp_now_init() != 0) {    Serial.println("ESP-NOW initialization failed!");    while (true);  }  esp_now_set_self_role(ESP_NOW_ROLE_CONTROLLER);  esp_now_add_peer(receiverMAC, ESP_NOW_ROLE_SLAVE, channel, NULL, 0);  esp_now_register_send_cb(onDataSent);}void loop() {    // dummy data for the myData struct, replace it with actual data    myData.latitude = 1.72;    myData.longitude = 1.03;    myData.altitude = 13.0;    Serial.printf("Sending Lat=%.2f, Long=%.2f; Alt=%.2f\n",                   myData.latitude, myData.longitude, myData.altitude);    sentStartTime = micros();    esp_now_send(receiverMAC, (uint8_t *) &myData, sizeof(myData));    delay(1000);   // send again in 1 second}

ESP-Now Receiver

#include <ESP8266WiFi.h>#include <espnow.h>typedef struct struct_message {  float latitude;  float longitude;  float altitude;} struct_message;// *** following 3 settings must match transmitter's settings ***uint8_t mac[] = {0x82, 0x88, 0x88, 0x88, 0x88, 0x88};const uint8_t channel = 1;struct_message myData;// **************************************************************void OnDataRecv(uint8_t * tmac, uint8_t *incomingData, uint8_t len) {  Serial.println("Data received!");  memcpy(&myData, incomingData, len);  Serial.printf("From: %02x:%02x:%02x:%02x:%02x:%02x, ",                 tmac[0], tmac[1], tmac[2], tmac[3], tmac[4], tmac[5]);  Serial.print("Latitude: ");  Serial.println(myData.latitude, 6);  Serial.print("Longitude: ");  Serial.println(myData.longitude, 6);  Serial.print("Altitude: ");  Serial.println(myData.altitude, 2);}void setup() {  Serial.begin(115200);  WiFi.persistent(false); // turn off wifi persistent  WiFi.mode(WIFI_AP);     // ESP-Now receiver must be in AP mode  WiFi.disconnect();      // Do not connect to WiFi  wifi_set_macaddr(SOFTAP_IF, mac); // if using Locally Administrated MAC  if (esp_now_init() != 0) {    Serial.println("Error initializing ESP-NOW");    return;  }  esp_now_set_self_role(ESP_NOW_ROLE_SLAVE);  esp_now_register_recv_cb(OnDataRecv);}void loop() {}

Here are two of my blogs for myESP-Now doorbell project andRange and Response time test

YourreceiverMAC is currently {0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF}. This is not a valid MAC address. Get the correct MAC address of the receiving ESP8266, by writing the following code:

    void setup() {    Serial.begin(115200);    WiFi.mode(WIFI_STA);    Serial.print("ESP8266 MAC: ");    Serial.println(WiFi.macAddress());}    void loop() {}

You can get more info from this tutorial.https://randomnerdtutorials.com/get-change-esp32-esp8266-mac-address-arduino/. Please figure out the correct MAC address and update it in your code. If you want to make print PCBs for your project, here you will get some idea about the cost.https://www.allpcb.com/blog/pcb-ordering/pcb-cost-per-unit.html

• esp8266
• wireless
• tinygps