Overload + operator for integer array, What's difference between two function const and without const at right?

Question

This is class of array to access the array of IntegerSet. You need access to the pointer ptr of IntegerSet GetPtr();

You need access to the capacity of IntegerSet GetCapacity();

GetPtr() const to const IntegerSet class.

class IntegerSet
{
 private:
    int capacity;
    int nelements;
    int * ptr;
public :
IntegerSet(int  size);
~IntegerSet();
int * GetPtr() const {return ptr;};
int Getcapacity() const {return capacity;};
IntegerSet &  operator =(const IntegerSet & rhs1);
friend IntegerSet  operator +(const IntegerSet & rhs1,const IntegerSet & rhs2);
};

IntegerSet::IntegerSet(int  size)
{
ptr=null;
capacity = size;
ptr = new int [capacity];
if(ptr==null) cout << "error allocation"<< endl;

}
~IntegerSet()
{
if (ptr) delete ptr;
}
IntegerSet & IntegerSet::operator =(const IntegerSet & rhs1)
{
int capacity_=rhs1.Getcapacity();
int *rptr1=rhs1.GetPtr();
for (int i=0;i<capacity_;i++)
{
ptr[i]=rptr1[i]; // ptr is pointer of class IntegerSet 
}
return *this;
}
 //this function friend of class IntegerSet is extern ..
IntegerSet  operator +(const IntegerSet & rhs1,const int& rhs2)
{
int capacity_=rhs1.Getcapacity();
IntegerSet  temp(capacity_);
int *rptr1=rhs1.GetPtr();
int *tptr=temp.GetPtr();
for (int i=0;i<capacity_;i++)
{
tptr[i]=rptr1[i]+rhs2;
}
return temp;
}

what's difference between two function int * GetPtr() const and int * GetPtr(); ?