H1B gap, help: PJiang, xiaobeitu please

Anybody knows what will happen if there is a gap between H1B transfer from previous company to new company? currently I quit from the previous company, and my h1b visa will be terminated on 10-20-07, now I got a offer from a new company, and they told me they need 3 week to process my h1b transfer, I worry about if they cannot transfer my h1b visa before 10-20-07, what will happen to me? I will loose my h1b visa for ever? if a gap is allowed, how long is it?

BYW, my salary and befit from the old company cover to the end of this monthe( October ), and my I-485 is pending( family based).

Thanks and have a nice day!