[akshay@localhost tmp]$ cat file
121 300
122 345
124 567
127 234

[akshay@localhost tmp]$ awk 'FNR>1 && $1!=p+1{for(i=p+1;i<=$1-1;i++)print i}{p=$1}' file
123
125
126

[akshay@localhost tmp]$ cat file2 
121 300
122 345
124 567
127 234
112 222

[akshay@localhost tmp]$ sort  file2 | awk 'FNR>1 && $1!=p+1{for(i=p+1;i<=$1-1;i++)print i}{p=$1}'
113
114
115
116
117
118
119
120
123
125
126

awk 'FNR>1 && $1!=p+1{for(i=p+1;i<=$1-1;i++)print i}{p=$1}'

1. $1 is the first column from current input line
   
2. p is the previous value of the last line
   
3. FNR Number of Records relative to the current input file
   
4. so FNR >1 && $1!=p+1 is a condition : if FNR>1 and $1 is different than previous value +1, then
   
5. for(i=p+1;i<=$1-1;i++) loop from previous record value +1 to current record value -1, and print i which is missing
   
6. {p=$1} is executed for each lines : p holds current record's 1st column value ($1)