Provided the content is level-appropriate for your guild group, you’ll earn experience and gold for your guild.
To earn Guild Dungeon Challenge credit, the 5-player party must be composed of at least 3 guild members.[...] To earn gold for Guild Challenge rewards, a guild must be level 5 or higher.
So, to get gold and guild XP you have to be in a dungeon level-appropriate groupd (which you weren't), additionally to receive gold and XP your guild must be lvl 5 or higher. If one of the criterias is not met, you are still able to get the "Guild Dungeon Challenge credit" (aka guild dungeon achievement).
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Originally Posted by Cryonic
well all the members of that guild was lvl appropriate, the lvl 90 is not in the guild
If there are at least three players of one guild in a dungeon group, the whole group counts as guild group, not just the three people.