You cannot physically fit a whole SSL datagram (max size 16KB) into 8KB of RAM. SSL requires multiple passes over the data to (eg) decrypt and verify a datagram. At this point, you cannot use standard SSL at either the server side or client side.
On the subject of reliablity: a 8 bit uC running at 16MHz needs a long time to do the public key crypto required to set up the connection. This means you need a GSM data link to be continuously available for a longer period.
This is true, but if you're using GSM you have a much beefier processor handling the GSM side and there's no need to use a tiny microcontroller. Some of the GSM modules will offload the whole HTTP(S) request for you.
Edit: or you could get a Cortex-M0 with 32K RAM for $2.
as someone unfamiliar with their specific workload, but who has used such boards to do vpn/ssl stuff, no -- it's not a limiting factor unless trying to skim power requirements.
On the subject of reliablity: a 8 bit uC running at 16MHz needs a long time to do the public key crypto required to set up the connection. This means you need a GSM data link to be continuously available for a longer period.