Contamination on genome assembly
This could be organism-specific. We don't have a lot of info so far, so I would check a few more things:
Run something like FRC_align. Check if there's a clear signal between regions flagged as suspicious by it and your coverage graph.
Is it a eukaryote? Plant? Check where mitchondria and chloroplasts are on the plot. They will have different GC/coverage signals than the rest of your assembly, which is fine, expected even.
Worried about contamination? Run kraken2 on your raw data and see what you get there.
Run RepeatMasker and check if repetive regions contribute to the effect you see.
Finally, unless your specimen is low heterozygosity, haploid, or inbred, you probably have some effect from ploidy. There are numerous ways to check that. A self-self dot plot with MUMmer and looking coverage histograms are a good start. The idea is that you should see two peaks (let's assume diploid). One for the split haplotigs, and one for collapsed.
I checked the GC distribution of 45 genomes in your order. It's kind of all over the place, but there are some bimodal ones:

What does GC vs Length look like? Here's the one from the 45 genomes mentioned above: