the gain is called sensitivity on some models, but it's the same difference.
what i see is the same line(s) being drawn all the time regardless of if it's going down a slope like it shows you doing and getting into those zigzags that i think are weeds or logs or other structure, but it can only draw it all as zigzags. the line(s) stay the same at all times. this means to me that it's turned up to high or that's just how that one works.
there are major differences in the sonar engines being used in sonars. some are very basic and some are very elaborate.
if it could show a soft bottom it would show a lighter shade of gray on top of the darkest line, but it never does it in the image you shared with us. it also looks to be drawing weeds as zigzags, but always using the same lines to represent them and every thing else full time. from what i see in the manual and youtube, that's all you can get out of it.
heres some youtube info on it
if you want more detail than the image you shared and i don't blame you, you have to tune (adjust what ever settings it offers such as gain/sensitivity) that one to see if it can or not.
if not, then time to upgrade. it looks to me as it's a bottom end unit that will show the basics only. it has a 4 level gray scale. they don't want to talk about target separation in the specs in the manual either. it will be very hard to draw any detail with a limited drawing capability of 4 level gray scale.
page 6 of the manual shows more of how it can do compared to your image you shared with us. it has broken dots trying to show soft substances. it draws zigzags for weed structure. so that's the best it can draw is on page 6. tune yours to draw like that and that's what you get with it. it has inverse which just makes strong returns show as lighter shades of gray or the opposite of the normal gray scale.
the sonar you are trying will show what you need to know. that is the bottom and the fish. like any sonar does. it's a fish finder. it draws images of fish to try to tell you how big they are. i am not sure why you need to know what the bottom is, but if it's real important, you can use a camera. i can tell with my anchor too.
for what it's worth, i don't fish with fish finders in a boat any more because i cast towards the shore and the fish finder doesn't look there, so theres no point in it. only in trolling would it be of use and i haven't fished that way in years. it's too slow of a way to catch fish for me and i'd rather cast and fly fish.
the latest in open water sonar is color and chirp and 3d imaging and using two beams at one time in one transducer. the thing for me was out growing (wanting more from a sonar) the sonar i was using. i own 5 of them.
if you have to spend more on one, try to spend more than you think you should and do lots of research first. if you ice fish, you may want an ice sonar that works in open water too. some do, some don't.
this is an very entry level fish finder.
i'm not trying to be a downer, just as truthful as i can be with the information i have. try the simulator to see what it can do also.