you could use liquid but it is a problem because its, well, liquid, so it kind of gets deep into the pores and spreads out a lot. if you get the gel you just create a nice fat drop and it stays.
what i do is this:
put a good size dollop of glue on the piece i want to glue (usually a small branch or frag) and then dip that in a tuperware container with a little SW. the glue will go from clear to milky and some "skin" will float off. i then add glue again and do the same. then, i stick the little piece of rubble wherever i want it in the main tank. i know plenty of people who just do all the glueing in the display tank (no tupperware) and it doesnt cause them a problem, but i am usually acclimating anyway so i might as well use that water