Goplats seem to be the internal name used by the Keen: Galaxy engine for floating platforms where Keen can land to move around. Throughout the games, Goplats can be seen moving around in either a straight back-and-forth line or a complex path (the latter function is only present in Keen 5 and Keen 6). Sometimes, flipping a switch will active a Goplat or make it follow a different path. Some Goplats are stationary, falling down whenever Keen lands on them, although they'll get back up once he gets off of them. In Keen 5 and Keen 6, additional Goplats can be found exclusively on easier difficulties, where they serve to prevent potential deaths and make traversing levels easier; these Goplats remain still and do not fall even when jumped on. There are also Goplats that will move away from Keen if he tries to jump on them (using the pogo will not make these react, however), which only appear in Keen 6 (though, they do exist in Keen 5's code).
The name has often been used to describe Koraths, probably because one Korath is seen in the beginning of the Korath III Base. In the initial version of Keen 5, playing the level on Easy difficulty will lead to an error saying "Goplat moved to bad spot" in a few seconds. In this time frame, only the Korath can be seen with no actual Goplats around, hence the misconception.
"Goplat" is probably a portmanteau of the words "Go" and "Platform".
The Goplat's appearance changes in each game it appears in. In Keen 5, there are actually two unique kinds of Goplats!