Find all permutations $(a_1, a_2,...,a_{2021})$ of $(1,2,...,2021)$, such that for every two positive integers $m$ and $n$ with difference bigger than $20^{21}$, the following inequality holds:
$GCD(m+1, n+a_1)+GCD(m+2, n+a_2)+...+GCD(m+2021, n+a_{2021})<2|m-n|$. 
1. **Define the permutation and the difference**: Let \( (a_1, a_2, \ldots, a_{2021}) \) be a permutation of \( (1, 2, \ldots, 2021) \). Define \( b_i = a_i - i \) for \( i = 1, 2, \ldots, 2021 \). We need to find permutations such that for any two positive integers \( m \) and \( n \) with \( |m - n| > 20^{21} \), the inequality 
   \[
   \sum_{k=1}^{2021} \gcd(m+k, n+a_k) < 2|m-n|
   \]
   holds.

2. **Maximal \( b_i \)**: Choose an \( i \) such that \( b_i \) is maximized. Note that \( b_i \geq 0 \). Suppose \( b_i \geq 1 \). Then there exists a \( j \) such that \( b_j < b_i \).

3. **Choose \( d \)**: Choose a positive integer \( d > 20^{2100} \) such that \( d + b_i \) is a prime number. 

4. **GCD properties**: Note that 
   \[
   \gcd(d + b_i, d + b_j) = \gcd(d + b_i, b_j - b_i) = 1
   \]
   because \( d + b_i \) is prime and \( b_j - b_i \) is less than \( d + b_i \).

5. **Choose \( m \)**: Choose \( m > 0 \) such that \( d + b_i \mid m + i \) and \( d + b_j \mid m + j \). Then set \( n = m + d \).

6. **Inequality analysis**: We have 
   \[
   2d - 1 \geq \sum_{k=1}^{2021} \gcd(m+k, n+a_k) \geq 2019 + \gcd(m+i, d+b_i) + \gcd(m+j, d+b_j)
   \]
   Since \( \gcd(m+i, d+b_i) = d + b_i \) and \( \gcd(m+j, d+b_j) = d + b_j \), we get
   \[
   2d - 1 \geq 2019 + (d + b_i) + (d + b_j) = 2019 + 2d + b_i + b_j
   \]
   Simplifying, we get
   \[
   -1 \geq 2019 + b_i + b_j \implies b_j \leq -2020 - b_i \leq -2021
   \]
   which is impossible because \( b_j = a_j - j \geq 1 - 2021 = -2020 \).

7. **Conclusion for \( b_i \)**: Therefore, \( b_i = 0 \) for all \( i \), implying \( a_i = i \) for \( i = 1, 2, \ldots, 2021 \).

8. **Verification**: Now we verify that the identity permutation \( a_i = i \) satisfies the condition. Let \( m > n \) and \( d = m - n \). We need to show that 
   \[
   \sum_{k=1}^{2021} \gcd(d, n+k) < 2d
   \]
   for all \( n > 0 \) and \( d > 20^{21} \).

9. **Define \( c_i \)**: Define 
   \[
   c_i = \frac{d}{\gcd(d, n+i)}
   \]
   for \( i = 1, 2, \ldots, 2021 \). Since \( d \mid c_i(n+i) \) and \( d \mid c_j(n+j) \), we have \( d \mid c_i c_j (i-j) \) for all \( i, j \).

10. **Bound on \( c_i \)**: This implies 
    \[
    c_i c_j \geq \frac{d}{|i-j|} \geq \frac{d}{2020}
    \]
    for all distinct \( i, j \). Therefore, there is at most one \( c_i \) which is strictly less than \( \sqrt{\frac{d}{2020}} \).

11. **Sum of GCDs**: Thus,
    \[
    \sum_{k=1}^{2021} \gcd(d, n+k) = d \sum_{k=1}^{2021} \frac{1}{c_k} \leq d \left(1 + 2020 \sqrt{\frac{2020}{d}} \right) < 2d
    \]
    because \( 20^{21} > 2020^3 \).

Therefore, the only permutation that satisfies the given condition is the identity permutation.

The final answer is \( \boxed{a_i = i \text{ for } i = 1, 2, \ldots, 2021} \)