Coming off of Propecia. Can you just stop taking it, or do you have to come off gradually?

You can stop taking it and be evaluated by a urologist. It is likely however that you will have some hair loss by stopping the Propecia (finasteride). If you are not already using treatments I would consider minoxidil, PRP, or laser therapy.

There is no question that for a relatively small percentage of men Finasteride has serious adverse sexual side effects which should be considered before taking the medication. These effects may continue for some time even after discontinuing but in most cases will resolve over time. Of course, whatever benefits the drug had on hair loss will be lost in a matter of weeks/months.  If you do decide to quit, should you taper off or just stop? There is debate on this question but my advice is to just stop. Tapering off means that you will lower your blood level of the drug but it will still be present and interfere with your hormones. If you are trying to reverse its sexual effects, stopping is probably the best course of action. 

You can stop finasterid quickly .After stopping finasteride you have to check your hairloss situation for a 3-6 month period.If you feel a continuous hairloss you can start  again.

It's fine to just stop taking it- but it is always a good idea to talk it over with your doctor before making changes to your medication routine. It takes ~2-3 weeks for it to completely leave your system, at which point you'll begin to see your shedding resume (and should see those side effects subside if it is the Propecia that is causing them).

Be sure to speak to your physician about this. It is fine to come off finasteride quickly. Hair will starts shedding in a few weeks and significant loss of hair may occur by 6-9 months.