How to convert a non gaussian distribution into a gaussian destribution?
I recommend order normalization from R package bestNormalize, good info at:
https://cran.r-project.org/web/packages/bestNormalize/vignettes/bestNormalize.html

Below a short comparison of a few methods:

Best Normalizing transformation with 6283 Observations
Estimated Normality Statistics (Pearson P / df, lower =&gt; more normal):
 - No transform: 21.8352 
 - Box-Cox: 4.0105 
 - Log_b(x+a): 4.5358 
 - sqrt(x+a): 7.1273 
 - arcsinh(x): 4.5268 
 - Yeo-Johnson: 4.0185 
 - orderNorm: 0.9744 


It is not written anywhere, but logic says, that you should rbind train and test datasets before this normalization if you want to use all of its advantages.