we are considering a day trip from Barcelona to Girona. I am not very good with a hilly terrain. How would you classify Girona?

The main part of the city if fine, it's in a river valley, but if you are planning on exploring the Jewish quarter and visit the Cathedral, then yes, you will have to walk a little uphill and climb some stairs, but once you reach the plaza, there is an easier access to the Cathedral just off to the side. From there it is an easy walk to the Arab Baths.

We are on our way to Cataluña including Gerona, my recolation from a day trip is that it is
uphill in the JEwish quarter.

there is a good walk from the train station to the Town. It is uphill from there to the Cathedral.

As others have noted, it is hilly, especially in the old Jewish Quarter also known as El Call.

Keep in mind Lance Armstrong trained close to Girona...lol.

The new part of town is flat and modern and the medieval
part is on a hill with the cathedral at the summit so some
ascending is needed to get to the heart of the town. Well
worth it in mho.

I think that we would consider a day tour from Barcelona that includes Costa Brava. How do you think I would fare on this?

The medieval part of Girona and the cathedral are well worth walking uphill for.

Are you talking about taking a guided bus tour to Costa Brava? You could rent a car for the day to drive there yourself. We made stops in Tossa de Mar, S'Agaro & Calell Palafrugel on our way to Cadaques. You can read my trip report to read more about it, if you wish; just click on my name.

Have not been but would it be possible to take a taxi up then walk down?

I

I am talking about taking a guided bus tour from Barcelona. It is possible to go by train, but for this day I prefer to take a guided tour. The cost is reasonable. I am wondering if you are on a bus tour you eliminate some of the climbing.