I was experiencing the below error message. While searching the internet and poking around on my SharePoint farm for a possible solution to the error i was able to find the below solution.
The mount operation for the gatherer application [GUID] -crawl-0 has failed because the schema version of the search gatherer database is less than the minimum backwards compatibility schema version supported for this gatherer application. The database might not have been upgraded.
Go to the Central Admin and determine the status of the SharePoint DB's. A healthy state shows "No Action Required" for all DB's but in my case the majority of my DB's needed to be updated.
Finally run this command:
psconfig.exe -cmd upgrade -inplace b2b -wait
Upon completion of running this command everything read "No Action required" and I was able to run the SharePoint crawl once again.