Can a Power Xpert Gateway be installed in the enclosure of an Enclosed Meter?

Yes. The PXG900 can be installed in a multi unit enclosure.  It would be indicated in the catalog string by a "C".  For example, IQ150MA6511-2A4C, the C stands for communications. 

Note - There are some enclosed catalog numbers where the C stands for 600 Vac, so always check Vista or with the Customer Integrity Team if you have questions or need verification.