I designed 5 Google ad banners on photoshop ( first time creating animated gifs for Google Ad) and I wasn't aware of the requirements. The size of the banners were very big between 200 and 1500kb. I don't know how to optimize and reduce them on photoshop. I tried online with gif convertors/compressors but the image quality is decreasing greatly. What is the best way to reduce the banners to 15k0kb and keep the image quality. The banners are at 4fps, I've attached one of the banners for reference.
The image size is way too big in pixel dimensions. Generally, GIFs should only be a couple of hundred pixels wide/tall. They should be created at the size they will be viewed on a web page. Your image is 1250px x 2500px which is why the file size is so large. I suspect it has something to do with your export settings when you created the image initially since it's much bigger than you say it's supposed to be (300 x 600px).
Open the GIF in Photoshop CC/GIMP.
Convert to RGB (Photoshop CC should do this automatically when you open a GIF)
Rescale the image to 300 x 600px
Convert to Indexed colour - 256 colours is the best quality, enable dithering. (In Photoshop CC this happens automatically when you export an animated GIF using File > Export > Save for Web - use a large number of colours, again 256 is the best quality).
Export the animated GIF.
Here's one I did in GIMP. The file size is only around 72kb. Size 300x600px. You could get the file size smaller by using fewer colours in Indexed mode, but the quality will suffer.