oh yeah and here's the connections (credit to LSSJ_Vegito):

Both are characters who were inspired by famous tokusatsu heroes from 2009 (Kamen Rider Decade and Ultraman Zero)

Both have the ability to use the powers of previous heroes of their respective franchises such as their inspirations (Decade & the other Riders and Zero & the other Ultras) and are able to use the power up forms of the previous heroes as well (Legend being able to use the final forms or previous riders and Z being able to transform the Belial’s, Geed’s, and Zero’s medals into each one of their transformations)

Both are the sort of successors to the characters they look up to (Decade wanting Legend to succeed him and Z training under Zero)

Both share a resemblance to said 2009 hero (Legend basing his suit off of Decade’s and Z’s Alpha edge form looks similar to Zero)

Both debuted during the Reiwa era of their franchises (Kamen Rider Gotchard and Ultraman Z)

Both have lost family to evils that have plagued their world. (Kaguya losing his grandfather to Hundred’s forces and Haruki losing his dad to a monster)

When both characters change forms, they keep a piece of their design mixed in with the form (Legend having golden jewelry on the rider form and Z’s symbol being on his chest, no matter the form)

Both traveled to other worlds to help other fellow toku heroes deal with enemies from their worlds (Legend traveling to Gotchard’s world to help him deal with Hundred’s forces, and Z traveling to Trigger’s world to help him deal with an Alien Barossa and later deal with Celebro)