Telegram that you used as an example is not decentralized, it is completely centralized. If this forum works like telegram, it does not mean it is still safe as long you have to still send posts online. PMs and anything you want to send on this forum with data, you have to go online. This forum do not nead any app for now, what matters most that members are demanding for is the new forum software, maybe it is going to come with apps or not.
I guess the "Gmail offline" is available for both PC and Android devices assuming it's still in use. The forum offline version could exist in similar manner mostly for the sake of flexibility. You could either download the forum on your laptop/pc or on your android (or other mobile operating systems) devices.
I think Gmail business account should be able to do that, but you have to pay for it like subscription. You want people to also pay before they can be able to make use of the offline version? It is not an easy work, it will cost than you think.