If you are reading this article in the future, note that installing a GSI on the Nokia G300 in 2021 came with permanent trade-offs:

Even if the bootloader were unlockable, the question becomes: Who would develop for the Nokia G300? Custom ROM development requires developers to own the device. In 2021, the G300 was a niche, low-margin device sold primarily through carriers like Tracfone and Cricket Wireless in the US. Its Qualcomm Snapdragon 480 5G chipset, while competent, was not a flagship or a popular mid-ranger.

Developers typically gravitate toward devices with large user bases (e.g., Samsung Galaxy A series) or “developer-friendly” hardware (e.g., Pixels, OnePlus). The G300 sold in modest numbers to consumers who viewed phones as appliances. The time investment to build a stable ROM—kernel hacking, vendor blob adaptation, SELinux policy fixes—is immense. For a $200 phone, the return on that investment (in donations, clout, or personal satisfaction) is minimal. Thus, in 2021, no skilled developer stepped forward, and the XDA-Developers forum for the device remained a barren wasteland of “please port this ROM” requests.

In 2021, the only available software for the Nokia G300 was the stock firmware:

Here is why "Nokia G300 custom ROM 2021" is a bittersweet search term. You could not unlock the bootloader via official means.

In 2021, the only methods involved:

Verdict for 2021: Unless you were willing to use paid software and risk bricking your device, you were stuck on Nokia's stock ROM.

If you were looking for a custom ROM for the Nokia G300 in 2021, you likely encountered a significant hardware and software barrier.

Result: Because the bootloader could not be unlocked in 2021, developers could not install a custom recovery (like TWRP) or flash a custom ROM (like LineageOS or Pixel Experience).