I work on several python scripts and I do not like the fact that using the plake8 (I suppose this is the origin) imposes the line length so that in the gutter (after saving a file) I get indicators of warnings that the line is too long. I do only want useful warnings/errors mentioned in the gutter so that I will look at them. Now I do not do that since it is most of the time 'line too long'. So how can I avoid this error. I have already put E501 in different settings as to be ignored but in vain.
You need to find out which plugin is providing these. If it's SublimeLinter, you can easily disable these warnings in the settings.
When I check on SublimeLinter User Preference file, I find the following:
so I think it is pyflakes but I do not know how to ignore the line-too-long error.
You are almost certainly running into PEP-8 E501. That's the Python standard library's style guideline and as noted, it'll flag anything over 79 characters as being a style error. I believe both pylint and pyflakes have a configuration file that lets you turn off various style checks however you'll have to check the documentation for those tools, and apply to your own project situation to implement.
EDIT: I found this specifically which also mentions Sublime. I have not used this advice, but seems like it's at least a start in the right direction. https://stackoverflow.com/questions/30698377/how-to-let-pyflakes-ignore-some-errors