I tried to set a default value for a column "country" in a table, but the command Default [country] "GB" was not accepted by GuiXT.

Only single fields are possible in the "Default" command. But you can use the following coding to set the default into all table rows:

Set V[i] 1
label setcoldefault
  if cell[All positions,country,&V[i]] //is there another line?
  default cell[All positions,country,&V[i]] "GB"
  Set V[i] &V[i] + 1
  goto setcoldefault