From a txt file named "BAK1_gene_seq" containing just a string of DNA sequence, write a Python program that calculates the AT content and GC content of the whole gene.

Sample output: BAK1's AT content is 30%

The file is a plain text file that contains the gene's sequence:

GCCGGGTGCCGCTGGCACCTCTATGATCACTGGAGTCTCGCGGGTCCCTC GGGCTGCACAGGGACAAGTAAAGGCTACATCCAGATGCCGGGAATGCACT GACGCCCATTCCTGGAAACTGGGCTCCCACTCAGCCCCTGGGAGCAGCAG CCGCCAGCCCCTCGGGACCTCCATCTCCACCCTGCTGAGCCACCCGGGTT GGGCCAGGATCCCGGCAG....

Python program should be able to open the file, read it, and calculate the percent of A's and T's within the sequence, as well as G's and C's.