How Did The Thinker Get His Powers?

Per the original comic version of Clifford DeVoe, the villain actually gains most of his mental powers from a vital accessory. This O.G. Thinker was more of a criminal mastermind than a butt-kicking meta-muman. After leaving behind a career as an attorney, DeVoe realized that most of Gotham's villainous underclass were strong enough to do some real damage, but lacked the intellect to organize and strategize. He began his own career as a bad guy by marshaling these small-time evildoers and directing them to do his bidding. In the course of this operation, he was always on the lookout for technological items that might help him in his work. This is how he came across the Thinking Cap.

The Arrowverse version of Clifford DeVoe (Neil Sandilands), who appears on The CW's The Flash, was struck by the STAR Labs particle accelerator while wearing this canonical Thinking Cap. The Cap both protected DeVoe's human DNA from mutation, and focused the particle beam to enhance his already formidable intelligence. His only meta-human traits are mental, as a result. 

The Thinking Cap allows DeVoe to project his mental energy. This manifests in several distinct "powers" including telepathy, telekinesis, probability manipulation, power transferral, genetic manipulation, dimensional manipulation — even power absorption. He makes a serious bid for Most Dangerous Mind in the DCEU. 

Just how these skills will be harnessed by Gunn in The Suicide Squad is yet to be determined. Given the nature of the DC multiverse, Gunn's version of the character could bear substantial or negligible similarity to any of the preexisting versions — fractured realities offer endless possibilities. Regardless, The Thinker is certainly a worthy addition to the team.