Gibraltar - worth a day from Seville?

I've been to Gibraltar and found it interesting, and it's more than just a "rock". However I visited it en-route to Jerez and was very near the night before. In your case you are looking at almost 3 hour drive each way, so I don't know since it would be a much longer drive, but it's ultimately your decision. If you don't have a car, then I don't even know how long it would take by bus.

Gibraltar was worth multiple days for me. 3 nights - 2 full days plus two half days on arrival and departure. I explored the tunnels, WWII installations, and caves, hiked the Mediterranean Steps, enjoyed the town, and more.

That said, in your situation given the distance from Seville, I would skip the stop. I would further suggest that Seville - Cordoba - Gibraltar is not a good one day trip. It's just too far and too much travel to enjoy much of anything in either Cordoba or Gibraltar.

Cordoba is a pleasant one day trip from Seville. You could leisurely tour the cathedral (consider buying tickets in advance, preferred entry times do sell out), stroll the Roman Bridge, and explore the historic center.

Perhaps you have visited Spain, previously? That might explain avoiding Madrid other than as a point of departure. I love Madrid, but maybe you have seen enough of it. The rest of your planning is fine - Segovia, Barcelona, Sevilla, Granada, maybe Ronda. And then there is Gibraltar.
Yes, it is a long drive from Sevilla, Yes, is is not a Euro country (as in you will need British pounds - although credit cards are certainly accepted), Yes, it is a very big rock and Yes, one can see Africa from there, Yes, the monkey population is very annoying, Yes, fish and chips and British flags are to be found, everywhere,
In my opinion, every stop on your planned trip is so much "better" than Gibraltar, I would never (again) give up a day in Spain for this place.
So, from my perspective, "NO", is it not worth a day from Sevilla.

We drove to Gibraltar from Marbella in 2007 or 2008. Loved it. Going into the rock (tunnels) was fascinating. Planned to take the cable car up, but it was too windy, so we caught a ride up with a local "cabbie" who was quite experienced. He turned out to be a great guide. Played up the Barbary Apes (Macaques) and made it lots of fun for us and our (then) teen-aged children.

One other consideration: Morocco is just across the strait. We considered going there but did not. Could you work your plans to also take some time there?

Yes, fish and chips and British flags are to be found, everywhere,

You make it sound like it's a bad thing. Gibraltar is British so of course there are going to be Union flags aplenty and lots of fish and chips. The Gibraltarians are fiercely patriotic and proud of their British roots.

Cordoba has much more to see than just the Mezquita. I'd axe Gibraltar from this 5rip's itinerary and go to Cordoba instead.

You haven't mentioned Toledo either. Both Cordoba and Toledo are worth at least a couple of nights to allow time for atmospheric wandering.

@4ptelapp - believe you’re being sarcastic, but back in the late 1970s the border was never open due to a dispute between Spain and the UK. If one wanted to visit Gibraltar from Spain one needed to take a ferry to Morocco and another back to Gibraltar.

What ever you do, don't read the advise online about driving to the top.
After a harrowing skinny switchback road in an SUV we were turned away by a kind Brit.
Parking was a nightmare when we made it back down so we never did really see much of Gibraltar.

Personally, I would much prefer to see something else, like Cordoba, or la Cueva de la Pileta, which is an amazing opportunity to see prehistoric paintings in a small group. The only reason we went to Gibraltar was that my children really wanted to see it after reading about it. So I did it for them, but unfortunately, I did not realize that the only way to see the top was either pay €200 for a cab tour lasting under two hours, or pay for the cable car to the top which would’ve cost about the same. Based on reading another guidebook, it sounded like I could catch a taxi to the top, and it’s about a 15 minute drive under normal conditions, so I thought it wouldn’t be very expensive. But the taxi drivers informed me that they only want to do the tour. On the flipside, it was helpful as far the cabbie knowing where to take us, waiting for us at a few spots, and then leaving us at the World War I tunnel so we could work our way down and see the sites.

We waited 30 to 45 minutes each way at the border. We parked in La Línea de la Concepción and walked across the border.

So, if you really want to see it because you know something about it, or you want to see the tunnels, which are indeed pretty unique, it might be worth a day. But most likely your money and your time could be better spent elsewhere.