I think the cause is that the backslashes used in Windows path separators are not properly handled (escaped) when printing the ${embeddedMongoDbTempDir} variable via the gmaven-plugin plug-in. The actual line in the POM file is line 131:

println "[INFO] Extracting MongoDB distribution to ${embeddedMongoDbTempDir} and        ";

Unfortunately I don't have a Windows system at hand so I can't really help with providing a fix for this issue. In the worst case we could remove the message but I like it for the sake of understanding what happens during the build.