Piping gel is a type of edible adhesive. It's stronger than edible glue. That doesn't mean it is better than edible glue. Both products have their pros and cons. You'll find more information further down in the post.

Piping gel (same as edible glue) works best if at least one of the surfaces you are trying to stick together is still soft (not dry).

It is possible to stick two dried out parts together with piping gel but you need to let it set (you might need to hold the parts together or support them for some time, until the piping gel dries out completely). And the part you are trying to attach can't be very heavy.
4) To stick heavy (thick) strips.

Edible glue might not be strong enough. 

Piping gel sets fast (especially if you apply a very thin layer), that is why make all adjustments as quickly as possible. With edible glue you have more time to push the strip down or up if there's need.

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