Just use your brain and eyes and look at the tag. there is not one obvious instance of gudako in the 3 most recent pages of "gudao | ritsuka fujimaru". There also exists a separate "gudako" tag, so their name is a non-problem. The proposed tags obviously need to be merged. Renaming is a whole different question (I don't think it needs a rename and proposed merge direction / name is fine, but either way it can be dealt with separately)

The protagonist for FGO is named Fujimaru Ritsuka, Gudako is the nickname for the male version while Gudako is for the female. It would probably be better for clarity if the tag was separated to differentiate between the two.

Since Ristuka Fujimaru is the name of both Gudao and Gudako I'd suggest merging both into the tag "ritsuka fujimaru" but also keep the old ones so people can either search one or any of the two

Make it the other way round as it makes it easier to search incase some one searches for ritsuka fujimaru (if it's their name obviously)

I would say the other way, for clarity.