Need help linking header to its source file

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I have a C++ header file that contains some function prototype. I have another C++ source file that contains the function definition. Here is my CMakeLists.txt:

find_package(catkin REQUIRED COMPONENTS
  geometry_msgs
  rospy
  std_msgs
  roscpp
  sensor_msgs
)

include_directories(
  include
  ${catkin_INCLUDE_DIRS}
)

add_executable(Keys scripts/Controls/Keys.cpp)
add_executable(Navigation scripts/missions/navigation.cpp)
add_executable(GPS scripts/missions/gps.cpp)

add_dependencies(Keys ${${PROJECT_NAME}_EXPORTED_TARGETS} ${catkin_EXPORTED_TARGETS})
add_dependencies(Navigation ${${PROJECT_NAME}_EXPORTED_TARGETS} ${catkin_EXPORTED_TARGETS})
add_dependencies(GPS ${${PROJECT_NAME}_EXPORTED_TARGETS} ${catkin_EXPORTED_TARGETS})

target_link_libraries(Keys ${catkin_LIBRARIES})
target_link_libraries(Navigation ${catkin_LIBRARIES})
target_link_libraries(GPS ${catkin_LIBRARIES})

My header file:

#pragma once

#include <sensor_msgs/NavSatFix.h>
#include <ros/ros.h>

class Localization{
public:

    // Function Prototypes
    void receiver(const sensor_msgs::NavSatFixConstPtr&);
    double bearing(void);
    double hubeny(void);
    double pythagorean(void);
    double fuzzy(void);

    // Constructors
    Localization(ros::NodeHandle);
    ~Localization();
private:
    // Data types
    double latitiude;
    double longitude;
    double altitude;
    const std::string topic {"/mavros/global_position/global"};
};

void Localization::receiver(const sensor_msgs::NavSatFixConstPtr& fix){
    if (fix->status.status == sensor_msgs::NavSatStatus::STATUS_NO_FIX) {
        ROS_INFO("No fix.");
        return;
    }
    latitiude = fix->latitude;
    longitude = fix->longitude;
    altitude = fix->altitude;

    ROS_INFO("Latitude: %f | Longitude: %f | Altitude: %f", latitiude, longitude, altitude);
};

Localization::Localization(ros::NodeHandle nh){
    ros::Subscriber location = nh.subscribe(topic, 1, &Localization::receiver, this);
    ros::spin();
};

My source file for C++:

#include "gps.h"

double Localization::bearing(void){
    return 0.0;
}

double Localization::hubeny(void){
    return 0.0;
}

double Localization::pythagorean(void){
    return 0.0;
    }

When I tried to use catkin_make, I received the following errors:

/usr/lib/gcc/x86_64-linux-gnu/7/../../../x86_64-linux-gnu/Scrt1.o: In function `_start':
(.text+0x20): undefined reference to `main'
collect2: error: ld returned 1 exit status
quantum_drone/CMakeFiles/GPS.dir/build.make:112: recipe for target '/home/kannachan/drone/devel/lib/quantum_drone/GPS' failed
make[2]: *** [/home/kannachan/drone/devel/lib/quantum_drone/GPS] Error 1
CMakeFiles/Makefile2:1245: recipe for target 'quantum_drone/CMakeFiles/GPS.dir/all' failed
make[1]: *** [quantum_drone/CMakeFiles/GPS.dir/all] Error 2
Makefile:140: recipe for target 'all' failed
make: *** [all] Error 2
Invoking "make -j8 -l8" failed