Can be GPERx6 powered by 802.3at?

It should work.
(seemingly the GPERx4 actually needs bt, but the GPERx6 works also on af and at)

Sources (both not particularly reliable/verified):

https://www.galaxus.it/en/s1/product/mikrotik-gperx6-css606-1g-2gi-3sout-6-ports-network-switches-62798033

The MikroTik GPeRx6 CSS606-1G-2Gi-3S+OUT is a compact and powerful PoE switch specifically designed for outdoor use. With a total of six ports, including three SFP+ ports and two combo Gigabit/SFP ports, this switch offers a flexible solution for various networking needs. The switch supports a maximum port speed of up to 10 Gigabits per second over the SFP+ ports and 1 Gigabit per second over the RJ45 ports. Thanks to its smart-managed capabilities, users can efficiently configure and manage the switch. The GPeRx6 is ideal for applications that require a reliable and high-performance network connection outdoors and is well-suited for mounting on poles.

  • Supports PoE standards 802.3af/at/bt with up to 60 watts
  • Maximum port speed of 10 Gbit/s over SFP+ ports
  • Smart-managed for easy configuration and management
  • Compact design for outdoor mounting.