I think the reason people say she sounds like Gumi is she has the same/similar vocal range and tone. Theres been a common comment about how she sounds like Gumi Adult at times and its even not too far fetched to use her as a 11th extended voicebank.
The main reason she sounds like other Vocaloids tho, is because of the fact she is the first Vocaloid on sale with the V4 recording technique (despite being V3) she doesn't behave the same. So while she sounds like Gumi she doesn't behave like her at all, and she can only sound Gumi-like at best if you flat line her. Normal singing seems to make her drop the Gumi-like act, which is want Internet wanted out of her. Edit: The problem with the method is that at times Vocaloids now can behave like other vocals as a result.
Thats my explanation, I do agree with the Japanese producers that said she sounds like Gumi she can, but there is a lot of Vocaloids in a common vocal type in V3/V4. Chika's more interesting then a lot of the older vocals.