Using constants here is definitely a good idea. I would suggest something like 'BYTES_PER_GIGABYTE" as the name. Basically I think the constant should be used for the conversion of units, not for the value itself (hence not having the 'one' in the name).
Something like this, perhaps:
```javascript
if (estimate >= 1 * BYTES_PER_GIGABYTE) {
    ...
}
else if (estimate >= 512 * BYTES_PER_MEGABYTE) { // or 0.5 * BYTES_PER_GIGABYTE if you prefer
    ...
}
```