C
You scramble a tactical team together and assign them a pair of armored
Hawkei with mounted MGs, slightly modified to fight through the Stargate easily. You'll keep the gate open for as long as possible after they go through, and redial immediately in order to maintain a semi-live tactical link. Signal delay due to wormhole transmission is an unfortunate and unavoidable issue that inhibits true real time command.
2*
After the search and rescue team goes through the gate, the first sound you get back via the audio feed is panicked gunfire, interrupted by explosions and the whistling sound of some kind of unidentified artillery. The search and rescue team was ambushed, that much is painfully clear, and it doesn't sound like they're doing particularly well for themselves. The optically feed from the Hawkei vehicles is muddled garbage, and one of the seems to already be on fire, but you can clearly see armored humanoid attackers firing bolts of light at your men from some kind of long rifle. It's less clear, but you could swear some of them have metal wings.
2*
Despite your best efforts, your men are crumpling up around the gate. The unfamiliar energy weapons do a number on vehicle armor designed to defeat conventional small arms, and your men are forced to cower behind the burning vehicles. The ranking officer on the ground is already dead, along with half of the tactical team, and the lieutenant who seems to have inherited the chain of command is frantically pleading with you to shut the gate down so that they can dial out and retreat.
1Of course, if you do that, it's possible the enemy will learn the address of Earth as the soldier punches it in...
A - Rally the troops and order a renewed assault! Being surrounded just means you're able to attack in any direction!
B - Order the lieutenant to lay down arms. Perhaps they'll be spared for long enough that you can negotiate their release.
C - Shut down the gate and prepare the gate room to receive a direct retreat and a lot of hostile contact.
1 For those unfamiliar with Stargate, the wormholes generated are 1-way only. So if you dial from earth to somewhere else, you can't travel back to earth until the wormhole closes and you redial from your current position back to earth.