Fill the gap between shower wall and splash guard

I have to tell you that that product is just not made for the application on the type of tub and tub surround that you have. If you were to try to start cutting it to fit against the tub corner and to the surround above that you would cut away the half inch edge of the splash guard leaving just the thin part to try to seal to the wall.

If I were you I would use a different approach to this. Secure a suitable sized piece of 1/2 inch thick Lexan plastic to use for a splash guard. Then cut that to fit against the form of the tub and surround. This will give you a full 1/2 inch bonding surface on all the edges. Then you can use a silicon sealer or RTV to secure that in place.

The picture below is an attempt to show you how to mark the Lexan piece so that know where to cut it.

A small block with parallel edges (red lines) can be used against the surround and tub sides (white lines) to trace a marking (green lines) over onto the Lexan piece (blue lines).

Some folks may want to use thin cardboard to create a cutting template to get the shape correct. You can still use the parallel sided block technique to transfer the shape to the cardboard.