No, they would be gatekeepers if it wasn't possible to get a phone which didn't run their operating systems. You can, it's just that they suck and nobody wants it. You have cause and effect backwards.

If you have a market with a handful of companies producing good products, and a handful of companies producing shit products nobody wants or buys, you cannot claim that the companies producing the good products are "gatekeeping", and that's the reason why nobody buys the shit products.

Them making the only phones anyone uses makes them the gatekeepers on what people do on their phones -> Apple and Google are gatekeepers under the DMA.

It doesn't matter how they became gatekeepers.