I am using Sharegate migration tool to migrate documents from Sharepoint 2007 to SharePoint 2013.After the migration I am not able to retain the document IDs and new document ids have been assigned in SharePoint 2013 for the deleted records.Could you suggest me how to retain document IDs. Thank you.
Sharegate does support the migration of Document IDs (related with the Document ID Service). please note that the feature has to be activated prior to the migration since it takes time to be all set.
If on the other hand, you are talking about the ID property (id assigned automatically by SP) then similarly to list item ids, it is not something supported by Sharegate since it would require accessing the SharePoint database to change the ID for each element.
There is a workaround to preserve these IDs while using Sharegate: https://sharegate.uservoice.com/knowledgebase/articles/248787-does-sharegate-support-preserving-list-item-ids-wh