Word2Vec how to treat words that aren't in the vocabulary
The words Going, Gone, Goes are considered to be similar in only one context i.e. they all have the same root word Go. This is known as Stemming/Lemmatization, both of which are special cases of normalization process. So, in order to assign some value to these words, you have to first lemmatize them. In order to get more details about this, I am attaching two links which can be helpful in your case:

http://textminingonline.com/dive-into-nltk-part-iv-stemming-and-lemmatization

https://stackoverflow.com/questions/771918/how-do-i-do-word-stemming-or-lemmatization