In general I'd say that an unusually high pH will hinder absorption, while some drugs might be more resistant to this, using Aluminum/ Magnesium or even Calcium carbonate should be avoided 1.5 hours before or 30 mins after.
Another problem with Gaviscon/ Peptac that probably outweighs the pH is the Sodium Alginate. It creates a sort of a slimy barrier that is particularly effective with GERD and/or ulcers, but It is very possible that it will hinder absorption for at least some period of time.
If the use of antacids with an Alginate is of a high priority, then looking into each drug separately would be in order. even a small window of time before taking your meds should help if a larger one isn't possible.
Published reports have clearly shown that weakly basic drugs which have low solubility at high pH could have impaired absorption in patients with high gastric pH thus leading to reduced and variable bioavailability.