Replace failed HDD in pfsense gmirror

[luckylinux]

Today I accessed pfsense interface and - by my surprise - I discovered the array was degraded.
I haven't received any email notification which I suppose it's quite strange.

Anyway I'll have to go fetch another HDD to replace the failed one.
How to perform this? I mean recreate partitions (possibly by copying the layout from the surviving disk), setup mirror, sync/refresh, …

Thank you for your help  ;)

[doktornotor]

Should really be as simple as

gmirror status (note which drive failed)
gmirror forget pfSenseMirror
gmirror remove pfSenseMirror ad?
shutdown -h
 <replace failed="" disk="">gmirror insert pfSenseMirror ad?
gmirror status (check if rebuild started)</replace> 

(Replace ad? with the failed HDD node.)