RGMII only requires a 125MHz clock; which is not particularly fast. It depends what you mean by "low-end", but you can certainly implement gigabit Ethernet on something like an Intel/Altera MAX 10 for example. It does rather depend on what you want to do with it though.

20 Euro Colorlight 5A FPGA boards from Aliexpress come with dual Gigabit Ethernet ports, open toolchain to build a SoC that can access them as well.

LiteX allows me to use it as a Gigabit Ethernet powered logic analyzer with a lot of inputs.

No, if the FPGA can do this, then it would be perfectly capable of interfacing to a gigabit PHY or a parallel/serial converter for a Gigabit SFP.

At the moment I have Artix 7 with ancient gigabit PHY running GigE. In Xilinx eco-system it is pretty low-end device.