From this page of the Returnal guide, you will learn how to open the Crimson Gateway.
Crimson Gateway in Returnal leads to the second biome of the planet Atropos and can only be opened with the Crimson key.
You will get the Crimson Key only after defeating Phrike - the first boss in Returnal.
A detailed description of the Phrike boss fight, as well as tips and tricks on how to defeat him, can be found on the Phrike - boss, how to defeat? page.
Once you have the key, return to the area with the gate, and it will automatically open and allow you to go to the second biome of the planet Atropos - Crimson Wastes.