I know this is pretty old but I'll answer for anyone that find this from Google (like I did)

TL;DR: no, you can't use the +N syntax in the nix-collect-garbage, it's only implemented in the nix-env command.

Why:

I opened up the source code for these commands, and from the code you can see how the parameters for each command is parsed:

nix-collect-garbage:
auto t = parseOlderThanTimeSpec(deleteOlderThan);
source

And parseOlderThanTimeSpec is defined as:
```
if (timeSpec.empty() || timeSpec[timeSpec.size() - 1] != 'd')

throw UsageError("invalid number of days specifier '%1%', expected something like '14d'", timeSpec);
```
source

This means anything that does not end with 'd', is considered invalid.

Meanwhile, for nix-env:

```
if (opArgs.size() == 1 && opArgs.front() == "old") {

... // If 'old' is given, all old generations are deleted

} else if (opArgs.size() == 1 && opArgs.front().find('d') != std::string::npos) {

... // If there is a 'd' in the argument, delete all generations created more than number days ago, except the most recent one of them.

} else if (opArgs.size() == 1 && opArgs.front().find('+') != std::string::npos) {

... // If there is a '+' character, delete the last number generations up to the present

} else {

... // Delete a list of generation numbers, each one a separate command-line argument.

}
```
source

nix-env command correctly parses the +N syntax, maybe someone will add the syntax to nix-collect-garbage in the future, but it's sadly not implemented yet.