retrofit convertor factory can not access GsonConverterFactory

Try to use same version for retrofit and converter-gson - 2.0.0-beta2. You are using beta2 for retrofit and beta1 for converter.

implementation 'com.squareup.retrofit:retrofit:2.0.0-beta2'
implementation 'com.squareup.retrofit:converter-gson:2.0.0-beta2'

Important note!

Retrofit change its package name since 2.0.0-beta3 version. Now you should use com.squareup.retrofit2. Here is example:

implementation 'com.squareup.retrofit2:retrofit:2.2.0'
implementation 'com.squareup.retrofit2:converter-gson:2.2.0'

In build.gradle (app) instead of:

implementation 'com.google.code.gson:gson:2.8.2'

write:

implementation 'com.squareup.retrofit2:converter-gson:2.3.0'