It could be like email, except with libsignal as its base. Federation definitely drove email to be as big as it is today, and it has made server reliability a non-issue (your server holds the message until the other server is online).

XMPP itself is huge, GCM is essentially just a frontend for it. Conversations is smaller than Signal in part due to Signal being around since 2011, and Conversations being created in 2014, and also Signal gaining many high profile endorsements, from Snowden to Hillary. Still, Signal is microscopic compared to Whatsapp.