Is iPad Pro's GPS supposed to be not as accurate as iPhone's GPS? I have an iPad Pro, and noticed that its GPS will tell the location off by about 100 feet to 150 feet. Sometimes it says it cannot find any GPS signal, when the iPhone 6 has no issue at all.

The spec page of iPad Pro says that it has 

Assisted GPS and GLONASS


at first I thought the aGPS is not as accurate as regular GPS, but then, it is the same specs as for iPhone 6 or iPhone 7.

So can it be fixed by software or will it require a hardware fix?

(if the GPS is to find a cafe or restaurant, it is no problem.  But when playing Pokemon Go, it is annoying to be not at the exact location -- except one advantage is that you can scan an area slightly off to the real location, so it is like you can scan 2 areas at the same time if you use both the iPad Pro and the iPhone).
WiFi-only iPads do not include a GPS receiver; only the Wi-Fi + Cellular models do. That is why at times you can’t even get a GPS signal at all.

But when there are Wi-Fi signals around or when you have an Internet connection, your iPad can use that to figure out its location.

You don’t need a cellular signal in order to get a GPS lock. I’ve gone on plenty of hikes in the boonies with no cell signal, and ended up with a perfect map. The “assisted” part of "Assisted GPS” only uses cell tower triangulation to acquire a quicker GPS lock.

I don’t see a reason why Apple would disable the GPS radio unless you activated the iPad on a carrier network, so you should be able to use GPS on an iPad + cellular without a data plan.