Using the multi-page converter from maven

This works:

asciidoctor -r asciidoctor-multipage -b multipage_html5 -D test test.adoc

But from the same terminal window, this doesn't:

            <plugin>
                <groupId>org.asciidoctor</groupId>
                <artifactId>asciidoctor-maven-plugin</artifactId>
                <version>2.1.0</version>
                <configuration>
                    <sourceDirectory>src/asciidoc/userguide</sourceDirectory>
                    <outputDirectory>target</outputDirectory>
                    <sourceDocumentName>master.adoc</sourceDocumentName>
                    <doctype>book</doctype>
                    <attributes>
                        <basedir>${project.basedir}</basedir>
                        <numbered></numbered>
                    </attributes>
                </configuration>   
                <executions>
                    <execution>
                        <id>output-html</id>             
                        <phase>prepare-package</phase> 
                        <goals>
                            <goal>process-asciidoc</goal> 
                        </goals>
                        <configuration>
                           
                            <backend>multipage_html5</backend>
                            <requires>asciidoctor-multipage</requires>
                                                     
                         </configuration>   
                    </execution>
                </executions>           
            </plugin>                       


[ERROR] Failed to execute goal org.asciidoctor:asciidoctor-maven-plugin:2.1.0:process-asciidoc (output-html) on project uom-manual: Execution output-html of goal org.asciidoctor:asciidoctor-maven-plugin:2.1.0:process-asciidoc failed: (LoadError) no such file to load -- asciidoctor-multipage -> [Help 1]

Any ideas?

The maven plugin does not use any of your local Ruby installation. It runs it's own "instance" within jRuby and all gems are contained within the different asciidoctor* jars.

If you want to use external gems, you need to use `gemPaths` to point to your Ruby installation path usually under your user directory, but can't confirm extact path in Windows, maybe it's within ruby installation path. BUT I'D RECOMMEND a fully self-contained Maven run that does not deppend on any pre-installation, just check this example https://github.com/asciidoctor/asciidoctor-maven-examples/blob/master/asciidoc-to-revealjs-example/pom.xml#L24, the steps are these:
1. Addd `mavengem-wagon` maven extension.
2. Add rubygems repository
```
    <repositories>
        <repository>
            <id>mavengems</id>
            <url>mavengem:https://rubygems.org</url>
        </repository>
    </repositories>
```
3. Add `gem-maven-plugin` configuration, same as in example. This will download the gems and place them inside `target`.
4. Add required gems (eg. asciidoctor-multipage ) as a `<groupId>rubygems</groupId>` dependency. Keep the example exclusions because asciidoctor gem is already contained in asciidoctorj jar.
5. Configure gemsPath and requires in the `asciidoctor-maven-plugin`: https://github.com/asciidoctor/asciidoctor-maven-examples/blob/ce3b5b9f97eb3245273d01d4daa86096abc7b45a/asciidoc-to-revealjs-example/pom.xml#L128-L131

Pay attention to match the path in `gem-maven-plugin` and `asciidoctor-maven-plugin`.