Why is the word けしゴム is half written in hiragana and half in katakana?
It just has to do with the words that it is made up of:
消し｛けし｝ comes from 消す｛けす｝which means to erase.
ゴム is a foreign word (usually katakana) meaning gum or rubber.
So you put the two together and you get the amalgam 消し｛けし｝ゴム.
Because けし comes from the verb and ゴム in itself is a loanword.