In reality, It is ok to apply for jobs when your are not gainfully employed but ONLY if you have good strong reasons for the gaps. e.g. Took time off to care for young kids, volunteer/charity work etc. Be prepared to back up the claims. You can put in the gaps in CV with a brief reason for the gap.

If you have no strong reasons or often just quit without a job so that there are gaps, then you will find you will have a harder time landing that next job. Ideally, quit only when you have signed a job offer.