I think we are all leaving out one important detail...
you have a 55 gallon tank, no job, and you are worrying about buying more fish that will outgrow your tank. To top it off, a 120 or 125 gallon isnt even going to be big enough for your longnose. They grow fast and will likely outgrow that tank too in no time.
A BGK may seem big enough that your gar wont eat him now, but like said, your gar is going to grow and will eventually be able to eat the knife. People keep them together with no problems, but every fish is different and since you have a gar that gets bigger than the average florida/spotted, then in your case it may be risky to try.