Both are sentai that give off a vehicle aesthetic, but are not actually classified as car sentai. Instead they both are about powerful treasures - Boukenger's "precious" and Kiramager's magical "Kiramei stones" respectively - and the team's search to recover them all.
Each also has a Silver Ranger from two different worlds who has personal ties to the villains.
Boukenger and Gokaiger celebrate the anniversary of the Super Sentai franchise. Their plots revolve around an adventure to search for valuable things: the Precious for Boukengers and the Greater Powers for Goukaigers.
It's the 29th sequel of the series from Super Sentai right after Mahou Sentai Magiranger which is the 28th sequel. If you would like to watch the super sentai series chronologically.
It's the 30th sequel of the series from Super Sentai right after GoGo Sentai Boukenger which is the 29th sequel. If you would like to watch the super sentai series chronologically.
It's the very first Super Sentai series what made many of it's successors possible. If you want to watch the original first season of the Super Sentai series. GoGo Sentai Boukenger is the 30th Season.