The oldest DIY gate opener is the mighty mule from GTO in the past few years commercial gate operator companies have recognized the need for competition in the DIY market. USAutomatic introduced the USAutomatic Sentry 300S is made in the U.S.A in 2009 for the Do It Yourself installer. Both the sentry 300 and FM-500 are bolt-on and do it yourself installation without welding. The mighty mule FM500 only has a 12 month warranty vs. the Sentry 300 10 year limited warranty.
The American made USAutomatic Sentry 300 is far superior when you want a solar power gate opener. Sentry uses a single 5-watt panel for single gates and dual gate systems. The Mighty mule FM500 says use a 10 watt solar panel for dual gates and 5-watt solar panel for single gates, but only provides minimal cycles. Both Gate systems come with components pre-installed in the control box, the US Automatic Sentry 300S does come with two 2 button remotes while the mighty mule only comes with one.
However, the mounting brackets provided with the mighty mule limit the installation and additional brackets might have to be purchased. The Sentry provides a truly universal bracket allowing for push to open or pull to open installations. The Sentry 300 is made to work on almost any gate and the speed of operation will not vary according to gate size.
Both gate systems qualify for the solar tax credit which can save you up to 30% off solar units and I recommend going that route.