Puzzle ZGWK 

Workings

Hint
Remove the spaces from the sequence first.

Answer
1301311 or 1210121.

Reasoning
Removing spaces from the original sequence gives:

   1 23 124 1251 26127 128129

   123124125126127128129

Now adding spaces back in, in the correct place:

   123 124 125 126 127 128 129

The next few terms could be:

   130 131 132
   or
   1210 1211 1212

Add these to the end:

   123 124 125 126 127 128 129 130 131 132
   or
   123 124 125 126 127 128 129 1210 1211 1212

Remove the spaces again:

   123124125126127128129130131132
   or
   123124125126127128129121012111212

Now break the sequence into chunks of increasing size:

   1 23 124 1251 26127 128129 1301311
   or
   1 23 124 1251 26127 128129 1210121

So the missing term is 1301311 or 1210121.