There is no legal basis - link completion or not.
There is no evidence of any gas! Just assertion and indication of "something".
Read through the very short unclassified assessment and it basically says "we know" and "trust us". Not one bit of something concrete or independently verifiable. Some circumstantial observations in a battlefield that may mean this or that. This sentence especially makes we very suspicious:
"We have identified one hundred videos attributed to the attack, many of which show large numbers of bodies exhibiting physical signs consistent with, but not unique to, nerve agent exposure."
The issue here is that exposure to a nerve agent like Sarin should show certain symptoms that the people in the videos do NOT show. Sarin "closes the pupils and opens the anus". In those videos all people seem clean, none have defecated, none have thrown up. That is inconsistent with chemical weapon effects and not mentioned in the assessment.