I took a frag of gsp and glued on the back of my tank (black background) about a year ago.... it now covers nearly 1/3 of my 5 ft tank. As it grew it spread to my sandbed and to a rock i purposely put in its way to collect and control spreading (btw once it starts taking off it will be EVERYWHERE) but back to your question... i found that the gsp growing on the vertical back wall had longer flowing “tenticles” than the one that had now covered the rock mentioned above.... now taking into consideration lighting and flow differentials between the gsp on the back wall vs the rock on the bottom i have noticed that the gsp tentacles on the vertical surfaces seem to grow noticeably longer than the ones that are “forced” to take direct light. So IMO if u want longer tentacles on your gsp, force them to grow on a vertical surface so the arms have to “reach” to grab the lights.... i will post a pic to show what the difference is in my tank, i happen to like tighter packed shorter tentacle for a standalone piece... but the gsp on the back wall look realllly cool as it sways with the currents in the tank.... pics to come... also this tank is a mixed sps, lps, softie, zoa/paly with fish. The gsp has been extremely hardy from day 1 dealing with lots of tinkering on my part. I would be more than happy to send you some of mine at no charge so you can “tinker” as well