Gabapentin 300 mg same as lyrica 50mg?

No lyrica (pregabalin) and gabapentin are not the same.

Pregabalin is described as (S)-3-(aminomethyl)-5-methylhexanoic acid.
Gabapentin is described as 1-(aminomethyl)cyclohexaneacetic acid

Gabapentin is used to treat post-herpetic nerve pain and epilepsy.
Lyrica is used to treat anxiety, nerve pain, fibromyalgia and seizures.

See: https://www.drugs.com/lyrica.html
https://www.drugs.com/gabapentin.html for more information.