Okay, you don't seem to get where Im going, so I'll spell it out more clearly.
You believe that a VBIED attack (or similar scenario) is probable, or has so great of an impact that even a small possibility of occurance should be guarded against.
Now to actually guard against this possibility is more than grabbing four guys off the back of a truck and telling them to stand guard until they are replaced. In the military even the simplist of tasks become a leviathan of process and SOPs within a short time frame.
Let's roll with your scenario. You seem fixated by 'the front gate'. Unfortunately for your scenario every base has more than one entrance, so why bother having a set of gate guards at one single point if the VBIED can be driven to a secondary entrance. Or they could use a set of bolt cutters to cut any locked gate. I've seen bases with a minimum of three entrances and some with more than twenty gates/entrances. So your manning is not '4 guys'. At a minimum it's 12 soldiers of varying rank, plus a senior NCO, plus an officer, plus at least one spare soldier, for a total of 15. But wait, there's more. Two soldiers arent enough to check every vehicle that comes through your gate during peak entrance or exit hours, nor are two soldiers enough to do a proper vehicle check and search of vehicles while observing the growing lineup of cars. if you've been to US bases, you'll see as many as 10 or 12 soldiers on a single shift, but we'll cap it at 4/day + 2/night, so now its actually 6 per gate for a total of 18 troops + officer + NCO + spare. Now you also need a vehicle to transport the troops around and for the officer and NCO to travel between gates. You also need an office with a secure lockup close to at least one of the gates. Your going to be doing this for a while so a modular tent isnt secure enough for storing weapons and classified paperwork.
Now you have to get the troops. Is this going to be a 'more duty/more honor' type of work that everyone buys into and sees as important, or will it get viewed as a shit job for dumping offending soldiers for punishment. You really don't want this to get regarded as a dumping ground for sloppy soldiers who earn extra duties. In this case we'll take the high road and say that the CO and Ops staff see this as highly critical to base security and give it full support. That means it is not dribs and drabs from different units, you get a whole platoon who will do the duty, ensuring a high level of teamwork and professionalism among the troops. For how long? Let's call it two weeks. Hell its so important, lets make it a month. Great, everyone in the Batallion gets a turn. But wait - who gets screwed over during block leave and Christmas? Sorry troops, no leave for you, someone has to man the gate. Sorry mate, no machine gun course for you, it's your platoon's turn to guard the gates during training season. Sorry chum, no field exercise for you it's your turn to man the gate next month. But it's all good, everyone is happy to be doing such an important job.
Now you have to train the troops. The Training Section just became your best friend. In addition to all the other
annual qualifications that are planned out on an annual basis we now add 'Gate Guard Training'. Which has become quite complicated over the last few years. What does a soldier on gate guard do if someone films him and puts it on youtube? What does a soldier on gate guard do if a protester stands off of DND property and yells abuse or blocks vehicles? How do you stop someone from grabbing your rifle from you? What does a VBIED look like? What forms do we fill out? What information is required? How do we rotate troops so that they all have time to eat but still keep maximum manning? What are my rights as a soldier if I get sued by someone I pulled out of a truck during a vehicle search? At what point do I call the MPs? These are not questions for the NCO or officer, these are questions that a professional soldier on gate guard duty should know before he starts his first shift, or else he's just a numpty with a rifle and an incident waiting to happen. This training needs to be done annually, so you just added 12 training courses to the Training Section list.
So, I'll stop there. I had some fun writing this out, with the point being its not as simple as you think it is to just start up a guard duty. This is an example of the issues to consider and plan for. My advice would be that if you still believe that strongly that an attack is imminent and that your idea would save lives, then I dont understand why you're talking to us here. You should be writing up your proposal and asking for an interview with the CO or RSM and present it to them. But I can't say either way whether you'll get a recommendation for promotion or an ass-chewing...