At least in the testing I've done, it depends on what you mean by "fast enough for gigabit".
I need to caveat the below as I have not personally tested an "mvebu" device, though hope to have one with a Marvell Armada 88F3720, Dual-Core ARM Cortex-A53 @1.0GHz in the coming weeks. It has been reported that the mvebu-based routers perform well.
"Gigabit" means gigabit downstream and no SQM and no VPN
Yes, there are likely moderate-priced, all-in-one devices that can perform the NAT at these rates with software flow offloading. OEM firmware may provide these rates, but has significant drawbacks in lack of SQM, VPN support, along with security and privacy that should be questioned.
"Gigabit" means gigabit symmetric and active at those rates and no SQM and no VPN
It seems unlikely to me that all-in-one devices can handle these rates with open-source firmware. From what I've seen, x86_64/AMD64 is required (recall that I haven't tested mvebu-based devices yet).
"Gigabit" means gigabit downstream and SQM (or other processes that require packet inspection)
From what I've seen, x86_64/AMD64 is required
"Gigabit" means gigabit WireGuard rates.
You're squarely in the realm of mid-range x86_64/AMD64 here, or an off-router device that can handle those rates.
"Gigabit" means gigabit OpenVPN rates.
Even an upper-range, consumer-targeted x86_64/AMD64 doesn't easily achieve these rates (if at all) with OpenVPN.