Do I need invitation for transit Visa for China? And on G type how many days I can stay in China

Hi,
I am Pakistani. I got HK double entry. I am planning to visit HK via transit through China for few days Then enter HK stay one week then again transit through China and enter HK for second entry and then back to my country. My route will be as follows
Pak-Shenzhen-HK-Shenzhen-HK-Pak. Can I apply for Transit G for China? If yes, how many days I can stay in China? For G do I need any invitation?
Regards