I'm confused about gas cooktops but no ovens...If it were me, I would push the contractors (I am reading that it's a new place that is being built?) for what I want. I can't believe that a gas cooktop is ok, but an oven isn't? I mean, they're plumbing for gas for the cooktop, how tough can it be to get the oven to match? It should be a no-brainer. I would certainly ask. (well, in my case, insist)
In answer to your question though, Convection is air that moves the hot air around the oven. I guess you could equate it with an automatic turntable in a microwave. It distributes the heat more evenly. Downside is you may not need as high a heat setting and it takes a bit to figure it out.
Regular heat eminates from a source and you can get uneven heating. Electric elements cool down reaaaaallllly slow so it's not an auto on/off like burners. Same with the oven. You can shut the heat source off (gas) and it's OFF. Open the door and it cools pretty quick. Electric elements stay hot so the cooling curve is a lot slower.
Plus an electric oven is much more dangerous. Ive had way too many burns that happen like instant cauterization. I've got scars from 20 years ago. Not being particularly negligent, just turning something or pulling something...and zip! There it is! Flames tend to warn you. You are close to them and the heat warns you before you get close enough to do damage. Not so with electric coils. If you're close enough to feel it it's too late.
The major problem with the convection ovens I dealt with is when you open the oven and get hit with this blast of hot air while you're checking on something.
I don't know if this helps or not. I'd still push about the 'why not?' thing about the gas oven.