No such geom: da0

<WDC WD3200BEKT-60V5T1 12.01A12>   at scbus0 target 0 lun 0 (ada0,pass0)
<MATSHITA DVD-RAM UJ-842 RB01>     at scbus1 target 0 lun 0 (cd0,pass1)
<Generic STORAGE DEVICE 0272>      at scbus2 target 0 lun 0 (pass2,da0)
root@ThinkPad:/mnt # gpart list da0
gpart: No such geom: da0.

The device is obviously recognised, but how do I tell what's on it?
GEOM works by "tasting". It probes every available device on the system for partition tables and metadata signatures such as geli(8) metadata. In the event it can't find anything that matches the known partition tables or metadata signatures the disk is empty in its view and that's why it says "no such geom". That doesn't exclude the possibility though that the disk does contain partitions and data, GEOM just doesn't recognize any of it.
It does contain data. Linux is able to identify partition information via either lsblk or blkid.
I can't really remember....

I think it was a Windows disk...

I was able to PXE boot SystemRescue and use GParted to resize the NTFS partition.