No, not at all. The challenge to unlock the Grau is 5 kills with an AR in 1 minute 25 times. This can be done in BR, easier to do in Plunder if you drop at a busy area. Far from impossible.
It is of course easier to do it in standard MP, but that does not mean its "making" you pay to get the gun.
They must have changed the Grau challenge as when it first appeared, I'm sure I had to just get 3 kills with AR's for 15 games with no attachments or similar.