How do I transfer Domain to another .gdb in ArcGIS?
You could export a featureclass that uses the domain from the original gdb to the new one and select transfer field domain descriptions from the 'Fields' section of Environments. The process is described in more detail here: http://support.esri.com/technical-article/000011973
Or you could use the "Domain To Table" tool (in ArcToolBox > Data Management Tools > Domains) to export the domain from the old gdb, then use the "Table To Domain" tool (located in the same toolbox as the previous tool) to load the domain into the new gdb.
If you are using ArcGIS Pro you can right click the original gdb in the catalog > domains.
Then click the domain that you are interested in highlighting the row. Then right-click the side of the table row > Copy.
Open up the new gdb table by right-clicking the gdb in the catalog > right click the side of the domains table > paste > save.