In order to understand my question, the following references must be studied.
There is an article that you might want to read, the article was written by Sebastian von Hoerner, "The general limits of space travel ", Science, 1962.
There is also a clear presentation of the basic ideas in Paul Nahin's book "Time Machines", in the technical note 6 "A high speed rocket is a one way time machine to the future".
Basically, this is relativistic analysis of space travel at 1 g constant acceleration (in the rocket reference frame).
If we assume that gravity modification effects can be achieved (big IF, but it is worth a thought), then we can assume that around the rocket (from all directions) the effect of gravitation is cancelled, except for a solid angle in the desired direction of travel. Only the matter in the universe contained in that solid angle will interact gravitationally with the rocket. In this case, the acceleration (in the rocket reference frame) will be inverse proportional to the square of the distance to the source of gravitation (approximations are necessary here, as there are many independent sources in that solid angle). The calculations made by von Hoerner have to be extended for variable acceleration (inverse proportional to the square of the distance to the source of gravitation). If you have a look at them, the calculations are not trivial and the differential equations that appear are not linear (this is an interesting problem, all by itself).
Another article that is worth a look is:
"The interplanetary transport network", by Shane D. Ross, American Scientist, May - June 2006.
This is basically an article about low energy transit orbits in the many - body problem.
The conclusions are directly related to my question. It seems to me that, if gravitational shielding effects become a reality, then space travel to the stars within our galaxy would be possible. In other words, Kepler - 186f (for example, situated at 500 ly from Earth ) could be reached in a human life span (in rocket years, due to time ditation), but it would take thousand of years (in Earth years) to bring that information back to Earth. Colonization is possible, extending our "sphere of knowledge", not so much.