I have seen several bakers in various online tutorials using piping gel to attach fondant decorations to their cakes. When searching online initially to see what brand of piping gel people recommend, I saw a lot of comments mentioning that piping gel never actually "dries" but just sets up a little tacky. If that is the case, will it hold decorations on the side of a cake ok without them slipping down the cake?

I used to use piping gel and then a mixture of water/gumpaste as a glue.

I've switched to candy melts as it dries SUPER fast and holds SUPER strong! Just melt a bit in a small bowl and use a brush.

It depends on how heavy the decoration is.  If your decoration is quite heavy, then I would use the candy melts for sure.  If they are smaller, piping gel is sufficient.  This is all personal preference and learning experience.  I have also found that different kinds of piping gel work differently.  If the gel is nice and thick, it can hold quite a bit of weight.  Sometimes I use a brush with a little bit of water to make the fondant stick to itself.

I'd use candy melts over piping gel any day. But if the decorations are light enough piping gel is okay, and if you don't have piping gel corn syrup is basically a thicker version. I think piping gel is thinned down corn syrup with some gums in it if I remember correctly.