Is there any alternative host available to run packet forwarder code, other than raspberry pi board??
Essentially any Linux system which can host an SPI peripheral can do so. This almost always means making a custom wiring adapter, unless you have a board that happens to have a pi-style header with SPI signals in the same place.
SBC’s based on various allwinner trailing-edge tablet chips, TI OMAP devices, and OpenWRT router chips have all been used. Many purpose built gateways use other embedded Linux SoC’s that are a bit less available at low prices on the small quantity market, but a bit more consistently available for designing into a product.
It’s that consistency of availability that’s often the issue for designs - the pi is a relatively poor choice, but it’s so well known and widely available that it ends up a bit of a default. If you put a lot of effort into designing something better, you have to worry about the platform you use going end of life just as you finally get everything solidified and ready to crank out in volume.
There are also concentrator cards that are USB interfaced via an MCU which offloads some of the tight timing work in a better way that the deprecated old designs based on SPI bridge chips. The may in some cases work in router type chassis with mPCIe slots, but there can still be compatibility issues. SPI based concentrator cards do not work in mPCIe slots, but only in boards that use an mPCIe connector for SPI rather than for the standard signals.
To add to cstratton’s answer: It depends on your budget 
Major industrial IoT manufacturer’s gateways come with the Semetech packet forwarder installed. Just a matter of enabling them.
Of course, if building yourself then go to stratton’s answer. BTW, from what I’ve seen, many manufacturers use embedded Linux in their LoRa gateways so you’d be following a well worn, albeit propreitary, path if you went that way yourself.
Personally I think that what is key is not just the packet forwarder, but being able to add custom services to the system image (and really to maintain it, eg build Linux from source). Some gateway manufacturers make this easy, with others its relatively straightforward but you’re really on your own, and a third group does nasty things like boot signing to specifically try to frustrate it.
That’s because the actual “LoRaWAN” aspect of a gateway is trivial; what’s really important for something field deployed is remote status and management.
What are you trying to do? The PI + a Hat is a quick and cost effective means of getting a packet forwarder up and running BUT right now the Hat is a little hard to find, Adafruit had one, bare without case, you can get the case from Aliexpress. A Nvidia Jetson Nano is a more reliable piece of hardware for a price and an over-kill hardware wise BUT they are going for twice or three times MSRP.