You could try calling Garmin tech support.
What kind of fish are you trying to catch?
The second pic looks perfectly normal.
To set the gain, change the range to at least twice the bottom depth. Manually raise the gain until a second echo starts to show at twice the bottom depth. Then set the range to whatever you want.
The GT51M (that's what you have,right? That's what you have it set for, I see) should show better fish returns than what you're seeing. There are a couple of fish in your 2nd pic @ around 6' that ought to have more definition. Try a master reset of your unit. Also, I hate to sound like a Garmin "tech" but do you have the latest software? I've seen a software upgrade make a sonar go from almost useless to "Wow! That's a LOT better!".
Ok here's a stoopid question from an electronics dufus:
How do you get the software updates from Garmin's website to your MFD on your boat?