"ERROR at line 1:
ORA-28545: error diagnosed by Net8 when connecting to an agent

ERROR at line 1:
ORA-28545: error diagnosed by Net8 when connecting to an agent

ORA-28545: error diagnosed by Net8 when connecting to an agent
 *Cause:   An attempt to call an external procedure or to issue SQL
           to a non-Oracle system on a Heterogeneous Services database link
           failed at connection initialization.  The error diagnosed
           by Net8 NCR software is reported separately.
 *Action:  Refer to the Net8 NCRO error message.  If this isn't clear,
           check connection administrative setup in tnsnames.ora
           and listener.ora for the service associated with the
           Heterogeneous Services database link being used, or with
           'extproc_connection_data' for an external procedure call.

Ok, I have several questions.

First, if your gateway is installed on Server 2, then you have no need of gateway related entries in the listener.ora on Server 1. Why have you created initGW_TO_PROG.ora on server 1? Why do you have a SID_DESC for GW_TO_PROG in the listener.ora on server 1? All that does is confuse the analysis of the problem.

Second, why is the listener on server 2 using port 1522 instead of the default of 1521?

Third, the error message "Unable to retrieve text of NETWORK/NCR message 65535" was particularly curious, so I googled ORA-65535 and found this:

ORA-65535: Oracle client cannot handle error code exceeding 65535. Actual error code and message follow:\n
Cause: Oracle server encountered an error whose error code exceeded 65535. Older Oracle clients (prior to version 12) cannot handle error codes larger than 65535.
Action: Actual Oracle error code and error message are a part of the error message for ORA-65535. Upgrade Oracle client to version 12 client libraries or higher.
(see https://docs.oracle.com/database/121/ERRMG/ORA-60001.htm#ERRMG-GUID-9B78A028-D760-4810-9CFC-9013FBD1FCC9).

Fourth, you state "SP2 is a DB link pointing to GW_TO_PROG service name.". No, "SP2" is not a db link, it is a stored procedure. There is a reference to a db link somewhere in the stored procedure. You've shown me all your config files but you haven't shown the actual code that initiates the call. If I were sitting next to you I'd want to see both the actual sql statement that references the db-link and the properties of the db link, as shown by 'SELECT * FROM DBA_DB_LINKS
WHERE DB_LINK = ''

The path from the sql statement making the call, to the target non-oracle database is a very clear an precise chain, as described in the article. To debug any issues, we simply follow each link in the chain. That begins with the SQL statement in your stored procedure.

Also, what version of OTG for ODBC is installed on Server2?