Looks like some (not all) gzip files are corrupted when attempting to download these on the firmware selector.
Specifically for the Raspberry Pi Compute Module 4 (64bit)- https://firmware-selector.openwrt.org/?version=21.02.0-rc3&target=bcm27xx%2Fbcm2711&id=rpi-4
Factory images are fine but the SysUpgrade image archives throw an error.
$ shasum -a256 openwrt-21.02.0-rc3-bcm27xx-bcm2711-rpi-4-ext4-factory.img.gz
a08aa421e10fb2f5d34ccbdb360fee50fd24e41b84550944702a90cd25574f5e openwrt-21.02.0-rc3-bcm27xx-bcm2711-rpi-4-ext4-factory.img.gz
$ gunzip openwrt-21.02.0-rc3-bcm27xx-bcm2711-rpi-4-ext4-factory.img.gz
$ shasum -a256 openwrt-21.02.0-rc3-bcm27xx-bcm2711-rpi-4-ext4-sysupgrade.img.gz
3bedc418f06fe280912c4943519597a8b3cbb3154bbacea7b7955ec470703d4e openwrt-21.02.0-rc3-bcm27xx-bcm2711-rpi-4-ext4-sysupgrade.img.gz
$ gunzip openwrt-21.02.0-rc3-bcm27xx-bcm2711-rpi-4-ext4-sysupgrade.img.gz
gunzip: openwrt-21.02.0-rc3-bcm27xx-bcm2711-rpi-4-ext4-sysupgrade.img.gz: trailing garbage ignored
Is there anywhere I can raise a pull request or something to help fix this?