question s about H1 transfer

I am H1(the 3rd year, just renewed),I-140 has been approved, now change employer.

1. Current job will be over at end of May, new job will start in mid August.Is it a gap? When should I start to file the transfer application?

2. Can I go back China in summer and apply the H1 transfer in China? Is it complicated than here?

3. The job deion on H1 application (I-129)should be same with the one when the new employer file perm for me?

Thank you so much!