Why had Gravis not checked for poison?
There were two main reasons for this.
The first reason was that he had not met any such "snakes" before. Jaimy could be considered one, but he had attacked him out of nowhere, which was more like a direct attack. Gravis had never been subject to such indirect schemes like poison. Sear had also been very convincing with his lies.
Though, that was not the most significant reason. The biggest reason was that Gravis knew that his whole being was so atypical, that probably no poison in this world could affect him.
Poison that attacked the Spirit? His Spirit was full of Destruction Lightning, the purest form of destruction that existed. Anything that entered his Spirit would be annihilated.
Poison that attacked his dantian? His dantian was also not filled with Energy. It was filled with pure Destruction Lightning, just like his Spirit. The same reason applied.