Returns the age-interpolated median of the GCDG references (Weber
et al, 2019). The working range is 0-4 years. This function should
be called when the `key = "gsed"`

or `key = "gcdg"`

.

## Arguments

- t
Decimal age, numeric vector

## Value

A vector of length `length(t)`

with the median of the GCDG references.

## Note

Internal function. Called by `dscore()`

## Examples

```
dscore:::count_mu_gcdg(0:2)
#> [1] 3.308477 49.244553 62.480171
```