My example indeed only works on Earth :) It was only an example.

We must be careful here as we are talking about two different things: gravitational interactions in general and gravitational effects that are the results of the interactions.

• The strength of gravity is what it is regardless of anything else. And it is very weak compared to the other interactions.
• Gravitational effects may in contrast be large because of the masses involved, and not because of the strength of the gravitational interactions (that is weak).

The magnitude of any resulting gravity effects is indeed given by the product of the strength of gravity and the involved masses.

I hope this clarifies ^^

PS: for the magnetars, I don't know, but I would check in this one.