I was thinking a strip because the gap between the cooler and the gpu is a 'C' shape. Looking at it from the rear of the case, you have the motherboard on the left side vertically. The motherboard mounts for the cooler and graphics card are solid and can't flex. It's providing a fixed point of space between the cpu cooler attached to the motherboard and the graphics card attached at the pcie slot, so the inside edge where the cooler hangs over the graphics card can't get any closer there. The motherboard itself is holding them apart. At the outer edge of the cooler where it hangs free over the graphics card is where it can flex and potentially touch the gpu. Adding the foam strip there would turn the "C" shaped space into a square. Giving a fixed separation at the long edge of the gpu between the gpu/cpu cooler provided by the motherboard and at the front long edge of the gpu separated by the foam strip.
I can see I'm having a hard time describing it and trying not to confuse here. Here's an image to try and illustrate what I mean. The yellow arrows show the distance between the cpu cooler and inside long edge of the graphics card, that's held in place (unmovable) thanks to the motherboard itself. The red arrows between the end of the cpu tower and the outer edge of the graphics card facing closest to you when you open the side panel is where there could be a little flex in the cooler, allowing it to get closer to the graphics card. There's nothing holding the two apart right there, it's the open part of the "C" shape of space between the two. (the cpu cooler being the top of the "C", the motherboard the edge of the "C" and the gpu forming the bottom of the "C" with nothing along the front edge). Having the foam strip there would support and keep a gap between the cooler and graphics card, closing in the "C" and offering spacing/support at the open edge of the gap between the two where the cooler may be hanging closer to the back of the graphics card.
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If a single strip of foam doesn't seem like enough, an inexpensive roll like that has 10ft so plenty to add an additional strip if you feel it's necessary. Just try to avoid using any more than you have to, the more foam you have covering the back of the gpu the more it may cover heat generating parts that need airflow to keep cool. The idea is to keep an open gap for airflow/cooling of the components at the back of the gpu.