Identifying whether training dataset is imbalanced in CART
Yes, your data is not balanced.

Before applying any kind of modeling it is suggestible to balance the data, you can use packages like SMOTE,ROSE to overcome the imbalance in your data.

For your next question, which index to use: It is subjective to your data. Please go through these links Link-1, Link-2 for better understanding but suggestible to try both Entropy and Gini Impurity. There is a good chance to get similar trees and similar outcome. 

Gain Ratio works better to reduce bias towards highly branching features like time, salary etc. (Information about the features of the dataset isn’t mentioned)

Do let me know if you have any additional questions.

Finally, how big is your test dataset?