The SQlite-based log appender that the gateway webpage uses is controlled in the logback.xml file in the data directory.
You should see a section like this:
<appender name="DB" class="com.inductiveautomation.logging.SQLiteAppender">
<dir>logs</dir>
<!--
Maintenance Settings
entryLimit: The maximum number of entries in the database. However, at any given time, there may be more than this number, due to how cleanup works.
maxEventsPerMaintenance: The number of event that can happen before a maintenance cycle occurs.
minTimeBetweenMaintenance: The minimum time (max frequency) between maintenance events. Takes precedent over max events.
vacuumFrequency: The number of maintenance cycles before a "vacuum" is performed, to recover disk space.
On disk, most log events are between 600-800 bytes.
<entryLimit>50000</entryLimit>
<maxEventsPerMaintenance>5000</maxEventsPerMaintenance>
<minTimeBetweenMaintenance>60000</minTimeBetweenMaintenance>
<vacuumFrequency>3</vacuumFrequency>
-->
</appender>
If you move the entryLimit tag out of the commented block, you should be able to define a different max event limit.