Returns the number of frames since the match started.
Example, called from post_draw.gml:
// draws a fire sprite on top of the player that animates at 10 frames per second. Using the gameplay timer as a base for the fire's image index allows it to animate at a consistent speed without creating a new local variable on the player:
if (on_fire) {
var fire_image = get_gameplay_time() / 6;
draw_sprite( fire_sprite, fire_image, x, y );
}