I have travelled about 50/50 OAT/GCT and the larger group sizes don't bother me at all. It didn't seem like I was lost in a crowd or anything like that.
As to tipping, it was endless, but I sort of expected that. I equipped myself with maybe $100.00 in $5.00 bills and used those. I just couldn't keep enough smaller Euros on-hand; many other members of our group did the same thing. Guides are in the travel business, after all, and with a pocket full of dollars, I'm sure they don't mind popping into a bank and doing the exchange. I really don't mind tipping and I noticed that our group seemed to be generous. I also tipped the hotel maids .... I tried to use Euros for that. We tipped our guide and bus driver at the end of the trip, and for that I used my larger-sized Euros. In total, I spent around $250.00 on tips, as I remember.
Food .... wow! ..... they provided us with excellent meals. Many days we had one meal on our own, but I was glad to back off with a sandwich or salad and a glass of wine. Our hotels were always located in great areas with tons of restaurants, cafeterias, and coffee shops. This was a very small expense; of course, I was not dining in expensive up-scale restaurants, but in nice small places. The very last thing I needed was more lavish meals! So, I spent maybe $10.00 or so for each meal on my own ..... twice I had only ice cream ..... hey! .... I was on vacation!
Frankly, I found the shopping uninspiring. The only things I bought were a few magazines, a lipstick and a pottery piggy bank. We went to the Lladro factory, but, while they are lovely, I just couldn't use any of the items.
If you are going in April, it could be cold and rainy. Carry an umbrella and some warm clothes. Don't just take shorts and t-shirts! My trip was in mid-May and we had snow. Long pants and a heavy hoodie shoud be adequate .... some in our group had to go out and buy them.
GCT seems to have put a lot of effort into creating a great trip. The provided meals, as I mentioned, are wonderful; the hotels are nice and in great areas. A special treat: the transfer days (Madrid to Valencia; Valencia to Barcelona) are fun, with stops in fascinating towns, such as Tarragona and Cuenca.
I do hope you go on the trip and enjoy it as much as I did!