Well. First of all. In my opinion it looks fine as it is. Probably more interesting than an uniform background, because as the yellow tint is iluminating her arm and the chest, it gives more separation.

But lets get into business.

I am also answering the aditional question you originally asked. Just be sure to stick evenly the gel in front of your flash. Use small pices of sticky tape.

I can only do basic controls in lightroom and PS

Well it is time you move further and make not so basic retouching.

• First of all mask the afected zone to protect the girl from harm. Do not mask only the gray part. Spread to some of the yellow zone.

Make it as a new layer and lock the transparency. Pick a color sample from the yellow part.

Use a wide soft brush, like 500px and paint using Color as the blending mode.

You of course expand your selection to the upper zone, but I wanted to leave that natural shadowy look.

This is not the most advanced method, but a basic one so you start to explore editing using layers on Photoshop.