You can see the linear-gradient for the caption
I notice that you have set only captionBackColor. You can set both captionBackColor and captionBackTopColor (both in the 'Color' section) to set the colors for the top and bottom of the caption area (which blend into each other in the middle).
If you do not explicitly set captionBackTopColor, then its default value of rgba(0,0,0,0) will be used and the captionBackColor (at the bottom of the caption area) will fade into a fully transparent area at the top. If you want such a gradient (a single color fading into transparency), then set either captionBackColor or captionBackTopColor to rgba(0,0,0,0) and the other to whatever color you like.
To see how two colors blend into each other, here is a sample gallery setting captionBackColor=rgba(255,0,0,1) (opaque red) and captionBackTopColor=rgba(0,255,0,1) (opaque green).
Also, your captionBackColor has been set to white (with a transparency of 0.3) and your gallery's background is also white. Setting a captionBackColor to the same color as your gallery's background means that it will not be visible (no matter what its transparency). (White with any amount of transparency will always look white on top of a white background.)
You can se the logo : even set at 50px height, when small window, the image go below the logo...
The extra height is likely to be due to padding used for the gallery title (which is usually text). The easiest remedy would be to increase the topAreaHeight (in the 'General' section) to allow more room for your logo image.
shows a few errors in the code
The errors reported by the validator are the height='50px' attribute in the <img> tag and no 'alt' attribute. When used as an attribute, the height should be numeric (with no 'px' suffix). A missing 'alt' attribute will fail validation but will not cause any problems. (This is unlikely to make a difference to the display. Most modern browsers will likely be tolerant towards such validation errors.)