Last year I replaced the fuel tanks in my 1984 Island Gypsy (twin Lehman 135s & outboard fuel & water tanks). The engine room layout is similar to the GBs. Moreover, I suspect the major project tasks are not really specific to the trawler brand & model.
When researching the project, I found that most seem to opt for cutting up and removing the old tanks, then replacing them with multiple smaller tanks, which are plumbed together. I found both pros and cons to that approach.
I opted to go with a less common approach: We cut approx 6'x3' access panels out of the hull sides, removed the old tanks in one piece, custom built new one piece tanks and slid them in from the outside. Then we re-laminated the hull and painted the top sides.
The advantages I perceived were: No need to remove engines and mess with many engine room systems along the way (which can be many man-hours and hence $ costs), no loss of fuel capacity, avoids multiple tank inter-connections (which would be potential points of failure), and finally, aesthetically the repainted topsides made her look new again.
Interestingly, the (fixed) price quotes for both approaches were within $2k-$3k of each other. On that size job, that's a minor amount. Neither approach is inexpensive.
I think the choice of approach would be heavily influenced by what yard skills are available near you. I have a yard nearby that had done this specific task before, who also builds custom (mid 40ft range) carbon fiber racing sail boats - so I was confident in their lamination skills. Some other yards in my area would not consider the surgical approach.
I just wanted to mention this approach as I didn't find it a common method for smaller trawler tanks. It's more often used on larger vessels (I understand it's often used to replace large engines).
If you'd like more info, I'd be happy to provide it.