Hit Command-Control-Escape. That brings up the classic desk accessories menu. Choose Control Panel, then hit Return. Choose System Speed, then, again, hit Return. You can then toggle it between the default "Fast" setting of 2.8MHz or Normal, which is 1MHz. Hit Return to go back to the main control panel menu. Depending on the ROM version of your IIgs, you may be able to hit the up cursor key to get to Quit, which is faster when the highlighted choice is near the top of the screen. ROM 3 machines require that you use the down cursor key. After choosing Quit, hit Return again. Choose Quit, and hit Return.
A IIgs can run everything the //e card in your LC can run. Which means that unless you're tight on space, might as well use the IIgs for your Apple II gaming purposes. The only negatives about either setup is when you're trying to run software that uses inverse capital letters in 80 column mode. Those letters will be replaced by the corresponding MouseText characters. Most games don't use those characters, luckily. It's usually office programs, like Microsoft Multiplan. In that case, an unenhanced //e, a ][ Plus, or an original ][ will be better.