Gatwick to Victoria Station one way with Railcard

Is your destination near Victoria? If not there are other choices.

Rather than booking 2T +1, when checking your fare just now the system prompted me that you can get a group save ticket for the three of you for £38.50.
This is a fully flexible, walk up fare, on Southern or Thameslink only, not GX.
No railcard needed.
Your fare seems to be advance fares, tied to a specific train.
That £38.50 seems to be the best fare, and is better than you would get on Oyster, which can't do Groupsave.
There is a debate on whether you can use Oyster to Gatwick, if you can it is not included in the capping.
The smart way and a lot cheaper is to pay to East Croydon, then tap in at East Croydon, which is in Zone 6, but not sensible with luggage.
Thameslink which serves a number of stations including Kings Cross St Pancras and interchanges with several tube lines may be more efficient at getting you to your destination.

Our accommodation is near Victoria Station.

The Express is about 40% more expensive and will save two minutes. Prices are fixed. Prices don't change.

The difference between Gatwick Express cost and the regular train cost seems minimal.

jgdh is wrong- Southern has advance fares from £13.50 to Victoria, which would make £40.50 on a fixed time, but the groupsave, on any ticket, already discussed, is cheaper and better.
GX is £65.70 for the 3 of you to Victoria- no group save discount exists, any train. No advance fares exist on GX. Without the groupsave discount the Southern fare would be £58.20 so £7.50 less. Also Southern is far more frequent than GX.
If you are needing the Two Together Railcard for other journeys then the fare for the 3 of you on Southern becomes is still £38.40 (any train, walk up) or £50.80 on GX.
Southern always wins by a country mile on cost and frequency. Whether you get a 2T railcard or settle for a groupsave for this journey depends on your other rail travel plans, but it certainly won't justify it's cost on this journey alone.

Thank you everyone for your advice.

It seems as though the best fare is £38.40 on Southern and I also see that those trains are more frequent and the length of the trip is pretty much the same. Will this be more crowded or have less room for luggage?

I did find a 9:10 Gatwick Express for £58.50. The 9:40 or 10:10 or 10:40 Gatwick Express trains were £43.35 , so a bit cheaper but more than Southern. It does say the £43.35 is groupsave ticket.

How crowded the train will be has a great deal to do with the time of day you are traveling. It will be more crowded during prime commuting hours. You may be overthinking this just a bit. I am a planner so I understand thinking through the options and having a plan in mind in advance. Just don’t let it bog you down.

When you arrive at Gatwick you will go through passport control which uses e gates which are quite quick. You then pick up any luggage and follow the clear signage to the train station.

I personally like the idea of having a ticket purchased in advance that will be good on any train that day.
I did this a few weeks ago and it was just a matter of a quick scan of the QR code off my phone and I was headed down to the tracks for the next train. I like easy when I know I am going to be tired from a long travel day!

I've been looking at another inquiry today on the same subject.
It turns out that the definitive answer is to use an Oyster card or contactless card. Every one will need their own.
Forget about the 2T railcard.
On Oyster/Contactless after 9.30am Monday to Friday, any time at the weekend, the fare is £9.50 on a Southern Train (not valid on Gatwick Express). At Victoria exit through the Platform 15 to 19 gateline to ensure that you are charged the correct fare.
If you are there earlier just wait for 9.30 to go through the barriers at Gatwick.