I created a custom table IBC_Contact_Gym :

IBC_CONTACT_GYMID   is   PRIMARY key,  contactid  + other fields

 If I do merge duplicate contacts  how do I merge the corresponding records from IBC_Contact_Gym the same way as SLX does for History, Activity tables based on contactid field ?

You simply change the ContactID field to your New ContactID......

UPDATE IBC_CONTACT_GYM

SET CONTACTID = MyNewContactID (Surviving 'Merge To' Record)

WHERE CONTACTID = MyOLDContactId (Bye Bye records).

Oh, and ModifyDate, ModifyUser of course....

SLX changes the ContactId and ContactName for History, Activity tables.... no records are actually 'merged' in these two tables.....

Address is another story....