Rather than starting by answering directly your question about Gondola trips, I will do a small introduction of Aveiro. I believe you will get a better understanding of what the city is about and you can decide by yourself.
Calling Aveiro the Venice of Portugal is an overstatement and somehow misleading. Personally I don't find it comparable in any way. I think it's actually a poor way of presenting the city. I have been in Aveiro and Venice. Certainly there are channels, but the amount and style don't compare.
Having this said, visiting Aveiro, as a day trip from Porto, is very well worth it. You can easily spend a couple of days in the region. Aveiro and it's surroundings have many attractions that hold by itself.
The boats are very nice and elegant, and are definitly one of the hightlights of the city. These boats are called "Moliceiros". Their original goal was to transport "moliço". This is essentially algae that was transported to the fields to be used as a fertilizer. Pay special attention to the drawings. These are scenes of daily life, most of times sarcastic, or involving some kind of sexualized image in a funny way. These boats are no longer used for transporting "moliço". So, if you choose to go an a tour, you should also think you are contributing for their preservation.
Near by, at a walkable distance from the city, you find an active saltern. Where salt is collected. It's very nice, specially if you get an explanation on the process of making salt. This also attracts a lot of wild life, specially birds. It's quite interesting if you enjoy this kind of atraction.
Aveiro is also very known for its architecture. You will find many "art nouveau" buildings.
Being a city by the sea, you will find many fish dishes and a very rich gastronomy. The city is also very well known for its traditional sweets made of eggs.
The Ria de Aveiro itself, is a very rich ecosystem. If you like nature, you will find certainly many things to discover and learn.
Not far from Aveiro you can find Costa Nova and its very particular houses. Very nice and different.
This is just a small sample of what you can do and visit in Aveiro. Although being an hightlight, the city is much more than the canals and the boats. Above all, don't expect to find Venice. You will be disapointed. If you do your research well, and visit it as something different you will certainly be surprised.
The boat trip is rather short (aprox 1h, and part of it going back and forth). I did it and I didn't find it particulary intersting. But I already knew the city reasonably well. Despite what I said, it's still a nice trip. You get a nice panorama of the city center and get a few explanations that perhaps you would miss otherwise. If you have the time, and the cost sounds reasonable to you, I would do it. It's a nice way to sit and relax while appreciating the city. If otherwise you prefer to explore on foot, you are in a tight schedule, and want to check/spend time in other atractions on the city and surroundings, I would skip it.
The trip covers aproximately this area. It departs aproximately from the round dot. In the city itself there aren't many canals. Outside there are many more, mostly connected to the "Ria de Aveiro".
The canals give a very particular charm to Aveiro, but With the risk of repeating myself, don't do your choices under the impression this is comparable to Venice.
If you zoom out on the city you can clearly see the lagoon and the many canals around it:
Extra tip: the city center is very flat. Aveiro was one of the first cities in Portugal, to adopt bycicles, as means of transport. I believe you can still get a bike for free to cycle around.