$ cat file1.txt
gene1   1234    78975
gene13  9876    ldlgfjk
$ cat file2.txt
gene1   abc
gene13  xyz
$ join -t '     ' file1.txt file2.txt
gene1   1234    78975   abc
gene13  9876    ldlgfjk xyz
$ awk 'FNR == NR { a[$1] = $2; next } { print $0, a[$1] }' \
> FS='\t' OFS='\t' file2.txt file1.txt
gene1   1234    78975   abc
gene13  9876    ldlgfjk xyz
$