Our VM with the custom-built kernel has the following settings.

Kernel 3.10.0-693.21.1.el7.x86_64, configured according to
https://cloud.google.com/compute/docs/images/building-custom-os#kernelrequired

[root@testvm ~]# grep "CONFIG_KVM_GUEST\|CONFIG_KVM_CLOCK\|CONFIG_VIRTIO_PCI\|CONFIG_VIRTIO_NET\|CONFIG_PCI_MSI\|CONFIG_STRICT_DEVMEM\|CONFIG_DEVKMEM\|CONFIG_DEFAULT_MMAP_MIN_ADDR\|CONFIG_DEBUG_RODATA\|CONFIG_DEBUG_SET_MODULE_RONX\|CONFIG_CC_STACKPROTECTOR\CONFIG_COMPAT_VDSO\|CONFIG_COMPAT_BRK\|CONFIG_X86_PAE\|CONFIG_SYN_COOKIES\|CONFIG_SECURITY_YAMA\|CONFIG_SECURITY_YAMA_STACKED" /boot/config-`uname -r`
# CONFIG_COMPAT_BRK is not set
CONFIG_KVM_GUEST=y
CONFIG_DEFAULT_MMAP_MIN_ADDR=65536
CONFIG_PCI_MSI=y
CONFIG_SYN_COOKIES=y
CONFIG_VIRTIO_NET=y
# CONFIG_DEVKMEM is not set
CONFIG_VIRTIO_PCI=y
CONFIG_VIRTIO_PCI_LEGACY=y
CONFIG_STRICT_DEVMEM=y
CONFIG_DEBUG_RODATA=y
CONFIG_DEBUG_RODATA_TEST=y
CONFIG_DEBUG_SET_MODULE_RONX=y
CONFIG_SECURITY_YAMA=y
CONFIG_SECURITY_YAMA_STACKED=y

[root@testvm ~]# modinfo ptp_kvm
filename:
/lib/modules/3.10.0-693.21.1.el7.nlsfourb.x86_64/kernel/drivers/ptp/ptp_kvm.ko.xz
license:        GPL
description:    PTP clock using KVMCLOCK
author:         Marcelo Tosatti <mtosatti@redhat.com>
retpoline:      Y
rhelversion:    7.4
srcversion:     A2A146848B24DA435A9360C
depends:        ptp
intree:         Y
vermagic:       3.10.0-693.21.1.el7.nlsfourb.x86_64 SMP mod_unload modversions
signer:         CentOS Linux kernel signing key
sig_key:        42:AB:F4:99:5E:5A:B6:FE:87:01:D2:A3:AF:BB:75:43:9C:3A:27:44
sig_hashalgo:   sha256

[root@testvm ~]# lsmod | grep ptp
ptp                    19231  0
pps_core               19057  1 ptp

[root@testvm ~]# modprobe ptp_kvm
modprobe: ERROR: could not insert 'ptp_kvm': No such device

userid@Users-MacBook-Pro:~$ gcloud compute instances describe testvm --zone=us-west1-a

Disks:
  licenses:
  - https://www.googleapis.com/compute/v1/projects/vm-options/global/licenses/enable-vmx
  - https://www.googleapis.com/compute/v1/projects/centos-cloud/global/licenses/centos-7

minCpuPlatform: Intel Broadwell

zone: https://www.googleapis.com/compute/v1/projects/myproject/zones/us-west1-a