I am not sure how you are using the term unemployed. Clearly someone with a paid academic position (e.g., adjunct teaching or lab tech) would not be considered unemployed. I am not sure if you consider an unpaid lab tech or a paid burger flipper as being employed. To me the real issue is being out of the field.
If you cannot get a relevant paid position and but can afford to be without income for a few months, then a year gap, and probably longer, isn't problematic. In fact many labs will hire unpaid researchers. In this case you could continue to conduct new research, publish, apply for grants, and gain new contacts. You could likely stay in an unpaid position as long as you could afford it without any affects on future job prospects. If you cannot get an unpaid research position then it really depends on how long you can milk publications from your past research and produce new research without any affiliation to a research group.
If you cannot get a relevant paid position and cannot afford to be without income for a few months you can sometimes find paid work in a related field. Working as a paid lab tech (e.g., washing test tubes) or adjunct teaching. These types of jobs won't help you publish more or get grants and in fact take time away from publishing, research, and getting grants. That said they can provide a limited set of new skills and cotnacts so are probably sustainable for a year or so.
In the absence of getting even a peripherally related job taking an unrelated job (e.g., burger flipper) even for a short period (i.e., months) gap can be problematic. Not only will it slow down publishing, research, and grants it may make you less flexible about being able to take up a new related position (e.g., how much notice would you have to give). You are also not building new skills or contacts.
A lot of the impact will depend on how important publication speed is in your field. If a few month delay in publishing will result in you being scooped, any gap is probably bad. Similarly the ability to do research without any resources will help you weather a gap. Similarly, if your field has new instigator grants with a clock that starts ticking upon graduation then gaps are bad.