It was mention in this thread http://forums.turbocad.com/index.php/topic,17996.0.html
that we have the option to use the GPU for "real time" renders in the Advanced Render Options. I ran a couple of test in TC 2016 to realize differences in render times.
I used a file I downloaded from 3Dwarehouse, since TC2016 RED is able to handle materials from Sketch UP drawings.
I first rendered the drawing in Advanced Render/GI Fine with standard options (use GPU unchecked). The render time for the drawing was 196s. If I went back to wireframe, or rotated the view, GI render time was still at 196s. During these times, CPU load was maxed while GPU load was at 40 - 60%
I then closed the file, then re-opened it. I changed to Use GPU, in the render manager for GI Fine. Note that after clicking this option the program seemed to freeze for a minute, but then came back. During this freeze, I could see that there was some loading on the GPU.
Next I rendered in AR/GI Fine (use GPU checked). The render time was now down to 48s. Better yet, is that when I went back to wireframe or rotated the view, GI render time was down to 23s. During this time, CPU load was around 25%, GPU load was maxed.
I think this option to use GPU is available in 2015, and maybe earlier, but I will leave to someone else to do the comparisons.
EDIT:: Whenever you have "Use GPU" set in the render manager, you won't see any progress bar or timer on screen. Everything seems to be done in the background (on the GPU actually) so it may seem like nothing is happening. We I go from wireframe to AR/GI, the display will stay in wire frame until the image is partially rendered (probably the GI phase) then will pop up again fully rendered. During these times, you won't get any info on your tool icons when you hover over them. But when you do, you know rendering is complete. I had to use a stop watch timer to come up with the render times stated above.
EDIT 2:: Note that the above testing was done on my PC which has a GTX 660 with 2G RAM. I tried the same drawing with GI_Fine Tone HDR, but my setup squawked, as GPU memory use went to 95%. Hence the testing with GI Fine. I would hope that a newer GPU such as the GT970 with 4G RAM would greatly improve those times.