Spoilers for Gokai 20 and 199 Heroes movie:
I liked episode 20. Not Gokaiger's best, but still good. I love how Hyuuga got to henshin into Gokai Silver, even if it was just in a dream. I thought it was cool how Hyuuga tested Gai. Pretty cool. Also cool to see Ryouma, even if it wasn't that big a part. The donuts were a nice comedic touch. I never saw Gingaman, so I don't know if it also works as a homage. (I was reading the Wiki page on Gingaman to get the spelling of the names right and I saw Ginga Yellow liked donuts. Question belatedly answered :laugh
Basco and Sari were cool, although still evil punks. Finally, I liked the Gingaman background music during the fight. All around decent episodes.
As for why Hyuuga didn't transform into Black Knight (Kuro Kishi) when he got the key when it's a plot point that is what happens when a warrior gets the key and proven in 199 Heroes, I think we need to remember the circumstances that each key went back to the warrior. When the Goseiger
got (ie: stole) theirs back, they didn't trust the Gokaiger to defend the planet since they just met and there was still a battle going on (It looks like. I just saw the special circulating on YouTube). In this episode, Hyuuga got the key back near the end of the episode and after both the normal-sized battle and Gai and the Gokaiger earning his trust. Hyuuga knew everything was find in the hands of the Gokaiger, so he had no reason to transform. Just my interpretation. I personally would love to see the past heroes henshin, but I'm okay with this format too. The writers are making it work.
Finally, if this is true, huge props to Teruaki Ogawa for asking Toei to bring Kazuki Maehara back for this episode. Granted, it wasn't a big part, but it's sure better than just (the also awesome) Hyuuga. :buttrock: to Teruaki.