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can you plant gaura in pots?

Gaura usually self-sows. Do you have seedlings coming up? It is not a long-lived perennial but allowing it to drop seed or gathering seed ensures continuation of the flower. In my patch, there are dead parent plants every year and seedlings are moved to fill any gaps. Gaura does not tolerate wet or soggy soil conditions in winter; soil must be fast-draining. If grown in a container, add sand or extra perlite to the growing medium. It is drought tolerant.

The taproot will constantly draw water from deep in the soil. Therefore, these lovely plants are drought-resistant. Remember to water very deeply, occasionally. Plants located in beds or in the yard or garden should be watered when the top couple of inches of the soil feel dry. Use a soaker hose or a slow trickle of water to water over an extended period to deliver deep, intensive watering.
I suspect this is root rot. Make sure the soil is well draining.
If the roots can recover the plant may survive.

https://www.gardeningknowhow.com/ornamental/flowers/gaura/growing-gaura-plants.htm

Gaura is a perennial flower for zones 5-9. If you live in northern Montana, it is too cold for Gaura to survive winters. It does not do well indoors and plants should be available in spring in zone 5. Gaura self-sows but transplant desired seedlings while they are only a few inches tall to minimize disturbance of the developing taproot. Here is a Montana map showing the hardiness zones. https://www.gardeningknowhow.com/planting-zones/montana-planting-zones.htm

This would be Gaura lindheimeri.

These article should help:

https://www.gardeningknowhow.com/ornamental/flowers/gaura/growing-gaura-plants.htm

https://www.gardeningknowhow.com/ornamental/flowers/salvia/growing-different-salvia-types.htm