DPI and Tunnelling Protocols - GRE

[Yellowfinuk]

Utilising DPI to detect application types such as tunnelling protocols
and GRE specifically in my respect. Is it typical - in a multi vendor
sense not just opendpi to deconstruct the GRE packet to reference the
application in the payload - then decide what to do with it at this
level or would they typically look at the fact it's a GRE packet and
apply rules based on that application level?

Doing some research for a work-based project and finding my way around
DPI.

Thanks in advance to any who reply.

kind regards
YF.