Guided Tours and tickets…

We are using Tours By Locals and seeing most of the Gaudi things as that is what is unique to Barcelona. We are doing a full day with a lady we have booked for Barcelona by Tours by Locals and the next day we booked her for a day trip to Montserrat.

We also have a history and Tapas tour thru the Gothic area booked the evening we arrive in Barcelona.

You will find that there is usually an audio guide available for significant indoor sights, so you have that as a possible third option. In some cases the audio guide is covered by the price of admission; in others, there's an extra charge of a few euros. This will usually be clear on the ticket-purchasing page of the official website. I have even found English audio guides offered in some not-well-known churches.

Most of the Gaudi sites in Barcelona are very crowded. I don't know how easy it is to keep up with a guide in such tight quarters; I've never tried to do it except on a tour of the British Museum in London, where it was a real challenge.

I am a fan of outdoor walking tours of the historic centers of towns or other areas with distinctive architecture. As of 2016 the tourist office in Barcelona sold tickets for a good Barri Gotic tour. It was easy to see that the size of our group was considerably smaller than the commercial "free" tour groups. There was also a modernisme tour that would be of interest to people who especially like that style of architecture. The modernisme tour didn't go inside any of the major sites (which would have been impractical due to timed entry tickets and high cost), but it gave us a bit of time in the small modernisme museum. Again, this was 10 years ago, and I don't know that free entry to that museum is still part of the deal.

Most worthwhile to book a guided tour: Sagrada Família first, because the symbolism and architecture make much more sense with expert explanation. Park Güell is nice guided if you love Gaudí, while the Gothic Quarter is best value if you enjoy history and hidden streets.
https://community.ricksteves.com/travel-forum/spain/guided-hot games-tours-and-tickets

Book official sites first; separate tour websites often cost more, but can help when official tickets sell out. My budget pick: guided Sagrada Família + self-guided Park Güell + wander Gothic Quarter on your own.

I tend to DIY, especially in places like Barcelona or Catalonia in general, because these places have been receiving tourists for decades and are very mature and well-set up to accommodate visitors. There’s information everywhere, in multiple languages, and the whole industry is very well developed.

But if you want to do one guided tour, I'd go with @hotgamesdotio's suggestion.

The tapas tour in the evening sounds fun, too! I will check out Tours by Locals. Can anyone vouch for the tours by Get Your Guide? We are wanting a tour that takes us to Montserrat and possibly a vineyard visit as well, which is something they offer. They have rave reviews. But I've never booked anything like this.

“Can anyone vouch for the tours by Get Your Guide”

Just to clarify…GetYourGuide is a tour consolidator. They are similar to Viator in that neither company runs tours themselves so you have a collection of different guides/companies under their umbrella.

My only experience with GetYourGuide has been in Paris. I booked a workshop, the artist contracted me and asked to change the day which didn’t work for me. I canceled (well before the time limit) and GYG immediately refunded my money.

I’m remembering that we had a guided tour of the Palau de la Musica, in Barcelona. It was a long time ago, but I’m pretty sure that at that time it was the only way to access the interior, other than attending a concert. The tour was excellent.