Correct frame bike for me I’m trying to measure a cyclocross bike. I’m 172 cm, leg length 77.5cm.

Some websites like ebicycles say that my bike frame size is 53cm.



I’m already looking for this bike with seat tube 54cm.



A gravel bike that can be easily a touring bike for long travels.

What would be the problems of getting this bike?

Thanks!
There is no such thing as 'your' bicycle seat tube size, except as a very rough approximation. Bikes of different models and from different manufacturers that are specified as the same seat tube length say can actually vary quite a lot in size. 

Seat tube length is a poor sizing system, and only really works to differentiate frames of the same model. This is why many manufacturers just use small, medium, large etc. The stack and reach system is much better, but somewhat hard for new riders to figure out an make use of.

All of that said, a 54cm bike sounds a bit to big for you. All of the bikes I've had are 54 or 55 and I'm a little taller than you.

Advice that is repeated by this community again and again is that you have to try out bikes for size before you buy. If you are buying from a local store, let the sales staff guide you. If you are buying used from private sellers you'll have to do a little research on bike fit and always do a test ride. If you are starting out and considering buying mail order from an Internet site - don't.