There is no one widget as such in GTK+ that does the spreadsheet style widget that you want. In Gnumeric it is more likely to be a custom widget that was created just for that application. This is what you will need to do for your application, possibly basing it on other widgets to make it easier for you. You could also take a look at the sources for Gnumeric to get ideas on how to do this.
If all you need is a Tree View to give a table for a result from say a data base then a GtkTreeView may possbile be OK with some modification to change styling. How you do this will vary depending on which version of GTK+ you are using and what you also want.
Thanks errol. I hope Gnumeric's sources will help me.