Honestly, it depends on how strict you want to define a "gimmick". Professional wrestling's roots in carnival shows mean that its original antagonist, the overconfident, out of town strong man waiting to be taken down a peg by the local tough guy, is a gimmick in itself. The strong man, the bone crusher, the limb breaker. These are all basic gimmicks.

By the beginning of the 20th century, professional wrestling had already broadly developed the concept of heels and faces which necessitated acting and exaggerating aspects of their style and personality to garner reactions. By the 1920s, with the establishment of the wrestling business as we would recognise it, with territories holding regular cards featuring wrestlers engaging in feuds with one another, the concept of the in-ring persona was cemented for good. In these early days, "ethnic dangers" were brought out as heels. Evil Arabs, Nazis, savages from outside of civilisation, et cetera.

The 1950s saw a real explosion in gimmicks with the television boom and the success of Gorgeous George's narcissist character. This is sometimes cited as the first truly modern gimmick, as we would know it.

Sources: Sex, Lies and Headlocks. Hooker (by Lou Thesz).