If I choose a new middleware home /u01/app/oracle/product/Middleware_12c.Then instance base directory should be /u01/app/oracle/product/gc_inst.
But /u01/app/oracle/product/gc_inst already exists for my 220.127.116.11 OMS.
When I upgrade from 18.104.22.168 to 22.214.171.124, then later to 126.96.36.199
each time I had to create a new middeleware home and base directory..
I ended up with these
188.8.131.52: /Middleware and /gc_inst
184.108.40.206: /Middleware12102 and /Middleware12102/oms (I put the base directory in the middleware folder and named it OMS)
220.127.116.11: /Middleware12103 and /Middleware12103/oms
11g to 18.104.22.168 is out of place upgrade which mean you will get a new middle-ware home. Starting 22.214.171.124 the gc_inct is outside the middle ware home.
So in 11g if you have -/u01/app/oracle/product/Middleware then i suggest you provide this during upgrade for new middleware home- /u01/app/oracle/product/12c/Middleware and inst home will be -/u01/app/oracle/product/12c/gc_inst