ga:daysSinceLastSession is a dimension. Dimensons can only be filtered on
- == Exact match
- != Does not match
- =@ Contains substring
- !@ Does not contain substring
- =~ Contains a match for the regular expression
- !~ Does not match regular expression !~
Answer: 'ga:daysSinceLastSession >= 20'
is an invalid use of a filter because you cant do >= on a dimension.
Link to filters with the Core reporting api: Filteres
Edit:
The only option I can think of would be a very scary filter something like ga:daysSinceLastSession==10,ga:daysSinceLastSession==11,ga:daysSinceLastSession==12
Note: I'm not sure how long a filter can be documentation says 128 characters for regular expression filters but I'm not sure if that limit will apply here. But the max length of a URL is 2000 characters so you will have to test the max length of your request string as you build it.