Yup, as i know most GR and LR qre quite friendly to everyone, but there still have some that will bark to strangers. I am not sure whether they train the GR/LR or the GR/LR likes to bark at stranger initially.
The problem is, you cannot expect that you MIGHT get 1 GR/LR that will bark to stranger cause it might not.

So, think properly. My fiend told me that Chow Chow is a good guard dog as well though it is a medium breed but they will bark at stranger (not sure whether will bite or not). Please consider what you need before you buy a dog.
I think mongrel will be a good guard dog cause they will bark at strangers but they might not listen to you as well. Hehe... At 1st when I plan to buy Choi Choi, i have the plan like you, hopefully she will bark at stranger. But, she did not and we decide to may be get another mongrel to be a guard dog loo since we cannot force her to bark ma.... They are friendly dog..

Hehe... Think twice before you buy...

Good luck.