fix(L1GraphTokenGateway): add a counterpartGateway getter for Router compatibility

Author: pcarranzav

contracts/gateway/L1GraphTokenGateway.sol

@@ -298,6 +298,16 @@ contract L1GraphTokenGateway is Initializable, GraphTokenGateway, L1ArbitrumMess
         return l2GRT;
     }
 
+    /**
+     * @notice Get the address of the L2GraphTokenGateway
+     * @dev This is added for compatibility with the Arbitrum Router's
+     * gateway registration process.
+     * @return Address of the L2 gateway connected to this gateway
+     */
+    function counterpartGateway() external view returns (address) {
+        return l2Counterpart;
+    }
+
     /**
      * @notice Creates calldata required to create a retryable ticket
      * @dev encodes the target function with its params which

test/gateway/l1GraphTokenGateway.test.ts

@@ -178,14 +178,15 @@ describe('L1GraphTokenGateway', () => {
           .setL2CounterpartAddress(mockL2Gateway.address)
         await expect(tx).revertedWith('Only Controller governor')
       })
-      it('sets L2Counterpart', async function () {
+      it('sets l2Counterpart which can be queried with counterpartGateway()', async function () {
         const tx = l1GraphTokenGateway
           .connect(governor.signer)
           .setL2CounterpartAddress(mockL2Gateway.address)
         await expect(tx)
           .emit(l1GraphTokenGateway, 'L2CounterpartAddressSet')
           .withArgs(mockL2Gateway.address)
         expect(await l1GraphTokenGateway.l2Counterpart()).eq(mockL2Gateway.address)
+        expect(await l1GraphTokenGateway.counterpartGateway()).eq(mockL2Gateway.address)
       })
     })
     describe('setEscrowAddress', function () {