Due to the way Google Compute Engine currently populates the virtual PCI bus it presents to VMs, sys/dev/pci/pci.c's pci_msi_blacklisted() blocks use of MSI. Since there is no 0:0:0 bridge device the existing workarounds there, such as the one for qemu, won't work.
Adding hw.pci.honor_msi_blacklist=0 to /boot/loader.conf on the images intended for Google Compute Engine will allow use of MSI, which is supported by the platform.
Efforts are underway to make this unnecessary, but are unlikely to be complete before 10.3-RELEASE.
This is no longer necessary. GCE now presents a Host PCI Bridge that FreeBSD recognizes as MSI capable.