The confusion here comes from the inconsistent usage of two unit prefixes, metric/SI and binary.
Technically correct would be:
- 1 GB = 1 Gigabyte = 1000³ byte = 1.000.000.000 byte
- 1 GiB = 1 Gibibyte = 1024³ byte = 1.073.741.824 byte
Unfortunately the term Gibibyte is not very commonly used, at least colloquially, and instead Gigabyte (or GB) is often used for both the metric and the binary unit, leading to slightly different numbers.
In your case, the Ubuntu Devices GUI reports the size of the file system as 134.2 GB whereas
lsblk and the Windows disk manager show 127.5 GiB for the partition size. Doing the maths on that:
Partition (lsblk/Windows): 127.5 GiB = 127.5 * 1024³ bytes = 136.902.082.560 bytes
File system (Disks GUI): 134.2 GB = 134.2 * 1000³ bytes = 134.200.000.000 bytes
The remaining difference of around 2.7 GB could be due to rounding errors and/or that the Ubuntu Disks GUI already subtracted some space for file system structures etc. from the total space.
You can get the exact partition size in bytes with e.g.
lsblk -b. That number should 100% accurately reflect the outer size of the partition, with no unit conversion or rounding going on and without taking anything about the contained file system into account.
The exact total inner size of the file system in bytes should be reported by
df -B1. It is slightly smaller due to the mentioned internal file system structures etc. taking up some space too.