It's really not a good idea to lower the gear at high speeds...very few aircraft can handle that, most will fail.
Best thing to do in a terminal dive, is back off the throttle and "flatten" the prop pitch, if applicable. Using elevator trim is also a good idea, as eddie suggested.
You can also work the stick, but some aircraft won't respond to "pulling" back on the stick, some respond to "pushing" the stick forward, which will cause the nose to climb.